refactor(stpmic1): use BIT and GENMASK helpers

Use BIT and GENMASK macros to ease stpmic1.h reading.

Change-Id: I808a62818d4188bb2f3686ab37518d369b6c41cb
Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>
1 file changed