fbdev: c2p: Use BUILD_BUG() instead of custom solution

Replace the call to the custom non-existing function by a standard
BUILD_BUG() invocation.
Suggested-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/20200112171521.22443-1-geert@linux-m68k.org

drivers/video/fbdev/c2p_core.h

@@ -12,6 +12,8 @@
  *  for more details.
  */
 
+#include <linux/build_bug.h>
+
 
     /*
      *  Basic transpose step
@@ -27,8 +29,6 @@ static inline void _transp(u32 d[], unsigned int i1, unsigned int i2,
 }
 
 
-extern void c2p_unsupported(void);
-
 static __always_inline u32 get_mask(unsigned int n)
 {
 	switch (n) {
@@ -48,7 +48,7 @@ static __always_inline u32 get_mask(unsigned int n)
 		return 0x0000ffff;
 	}
 
-	c2p_unsupported();
+	BUILD_BUG();
 	return 0;
 }
 
@@ -91,7 +91,7 @@ static __always_inline void transp8(u32 d[], unsigned int n, unsigned int m)
 		return;
 	}
 
-	c2p_unsupported();
+	BUILD_BUG();
 }
 
 
@@ -118,7 +118,7 @@ static __always_inline void transp4(u32 d[], unsigned int n, unsigned int m)
 		return;
 	}
 
-	c2p_unsupported();
+	BUILD_BUG();
 }
 
 
@@ -138,7 +138,7 @@ static __always_inline void transp4x(u32 d[], unsigned int n, unsigned int m)
 		return;
 	}
 
-	c2p_unsupported();
+	BUILD_BUG();
 }