MIPS: Make all extern-ed functions in bitops.h static

All these functions are expected to be static inline-ed.
This patch also fixes the following build warnings on MIPS targets:

include/asm/bitops.h: In function 'ext2_find_next_zero_bit':
include/asm/bitops.h:862: warning: '__fswab32' is static but used in inline function 'ext2_find_next_zero_bit' which is not static
include/asm/bitops.h:885: warning: '__fswab32' is static but used in inline function 'ext2_find_next_zero_bit' which is not static
include/asm/bitops.h:887: warning: '__fswab32' is static but used in inline function 'ext2_find_next_zero_bit' which is not static

Signed-off-by: Shinya Kuribayashi <skuribay@pobox.com>