pwm: qpnp: Fix wrong usage of ramp control address

Currently, the ramp control address is determined only by checking the
HW revision of LPG. However, that only applies to LPG peripherals of
LPG_CHAN subtype. Read the subtype as well so that the proper address
offset for ramp control can be determined and used.

CRs-Fixed: 733418
Change-Id: I404c66695eb8a9ab942b0a5d8a82c17b6016953a
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>