Blackfin: fix cache Kconfig typo

The Kconfig option is "BFIN_EXTMEM_WRITETHROUGH", not "..._WRITETROUGH".
Signed-off-by: André Goddard Rosa <andre.goddard@gmail.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>

arch/blackfin/kernel/cplb-mpu/cplbinit.c

@@ -38,7 +38,7 @@ void __init generate_cplb_tables_cpu(unsigned int cpu)
 
 #ifdef CONFIG_BFIN_EXTMEM_DCACHEABLE
 	d_cache = CPLB_L1_CHBL;
-#ifdef CONFIG_BFIN_EXTMEM_WRITETROUGH
+#ifdef CONFIG_BFIN_EXTMEM_WRITETHROUGH
 	d_cache |= CPLB_L1_AOW | CPLB_WT;
 #endif
 #endif