leds: leds-pm8xxx: add mask for brightness control

With existing mask in use, one bit of another functionality
of the register is at risk of being overwritten.  Add a new
mask to leave bit <4> untouched.

CRs-fixed: 426193
Change-Id: I04a9d3496ef9ac569523f0d48569b0beee1bb509
Signed-off-by: Amy Maloche <amaloche@codeaurora.org>