mfd: pm8xxx-spk: Fix gain value for speaker.

The speaker gain value is not being set properly.
Ignore the current value of speaker gain for setting the
new value as the previous gain is not relevant to the new
value to be set.

Change-Id: I347fbc4052ba479105abe46278763d3c8c27b8e2
CRs-Fixed: 390107
Signed-off-by: Asish Bhattacharya <asishb@codeaurora.org>