m68k: sun3: Improve Sun3/3x DVMA abstraction in <asm/dvma.h>

When building with W=1:

    arch/m68k/sun3/dvma.c:65:13: warning: no previous prototype for ‘sun3_dvma_init’ [-Wmissing-prototypes]
       65 | void __init sun3_dvma_init(void)
	  |             ^~~~~~~~~~~~~~

    arch/m68k/sun3x/dvma.c:178:6: warning: no previous prototype for ‘dvma_unmap_iommu’ [-Wmissing-prototypes]
      179 | void dvma_unmap_iommu(unsigned long baddr, int len)
	  |      ^~~~~~~~~~~~~~~~

Fix this by moving the declarations for sun3_dvma_init() and
dvma_unmap_iommu() to <asm/dvma.h>.  Avoid #ifdefs in callers by
providing dummy static inline functions.
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/ec93ed1b28b75674ae9a0b42de083a9bb140f0b3.1694613528.git.geert@linux-m68k.org

arch/m68k/include/asm/dvma.h

@@ -58,12 +58,16 @@ extern void dvma_free(void *vaddr);
 #define dvma_vtob(x) dvma_vtop(x)
 #define dvma_btov(x) dvma_ptov(x)
 
+void sun3_dvma_init(void);
+
 static inline int dvma_map_cpu(unsigned long kaddr, unsigned long vaddr,
 			       int len)
 {
 	return 0;
 }
 
+static inline void dvma_unmap_iommu(unsigned long baddr, int len) { }
+
 #else /* Sun3x */
 
 /* sun3x dvma page support */
@@ -78,9 +82,11 @@ static inline int dvma_map_cpu(unsigned long kaddr, unsigned long vaddr,
 #define dvma_vtob(x) ((unsigned long)(x) & 0x00ffffff)
 #define dvma_btov(x) ((unsigned long)(x) | 0xff000000)
 
-extern int dvma_map_cpu(unsigned long kaddr, unsigned long vaddr, int len);
+static inline void sun3_dvma_init(void) { }
 
+int dvma_map_cpu(unsigned long kaddr, unsigned long vaddr, int len);
 
+void dvma_unmap_iommu(unsigned long baddr, int len);
 
 /* everything below this line is specific to dma used for the onboard
    ESP scsi on sun3x */

arch/m68k/sun3/sun3dvma.c

@@ -20,18 +20,6 @@
 
 #undef DVMA_DEBUG
 
-#ifdef CONFIG_SUN3X
-extern void dvma_unmap_iommu(unsigned long baddr, int len);
-#else
-static inline void dvma_unmap_iommu(unsigned long a, int b)
-{
-}
-#endif
-
-#ifdef CONFIG_SUN3
-extern void sun3_dvma_init(void);
-#endif
-
 static unsigned long *iommu_use;
 
 #define dvma_index(baddr) ((baddr - DVMA_START) >> DVMA_PAGE_SHIFT)
@@ -274,10 +262,7 @@ void __init dvma_init(void)
 
 	dvma_unmap_iommu(DVMA_START, DVMA_SIZE);
 
-#ifdef CONFIG_SUN3
 	sun3_dvma_init();
-#endif
-
 }
 
 unsigned long dvma_map_align(unsigned long kaddr, int len, int align)