MIPS: constify address pointer in test_bit()

Fix several warnings when enabling UBIFS on MIPS:

In file included from ubifs.h:2137:0,
                 from ubifs.c:26:
misc.h: In function 'ubifs_zn_dirty':
misc.h:38:2: warning: passing argument 2 of 'test_bit' discards 'const' qualifier from pointer target type [enabled by default]
../include/asm/bitops.h:569:23: note: expected 'volatile void *' but argument is of type 'const long unsigned int *'

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>