regmap: Assign boolean values to a bool variable

Fix the following coccicheck warnings:

./drivers/base/regmap/regcache.c:71:3-18: WARNING: Assignment of
0/1 to bool variable.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Zhong <abaci-bugfix@linux.alibaba.com>
Link: https://lore.kernel.org/r/1611215961-33725-1-git-send-email-abaci-bugfix@linux.alibaba.com
Signed-off-by: Mark Brown <broonie@kernel.org>