mfd: pm8xxx-spk: Fix settings for the class D audio amplifier

Test results based on noise gate recommended the following settings

0x254=0xC8 (Threshold=110, preamp bias=01)
0x255=0xC1 (Hold=110, max attn=0000, mute=1)
0x256=0xB0 (decay=101, attack=10, delay=0)

CRs-Fixed: 385187
Change-Id: I7276a6d0689ffb475aa8c58214fd2b00fe62f81c
Signed-off-by: Asish Bhattacharya <asishb@codeaurora.org>