leds: qpnp-wled: Fix the APIs to read/write WLED registers

Fix the order of parameters passed into read/write APIs of WLED
driver to make it more readable. Fix the masked_write API to
directly use regmap API so that it can be protected properly to
avoid racing with secure writes.

While at it, remove the comments for variables that were removed
a while ago.

Change-Id: Ib372d3272550973fd98992f28b3b9bf7e5eb7f0f
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>