leds: qpnp-flash-v2: change from dev_*() to pr_*() for logging

Currently, dev_*() variants are used in the flashv2 LED driver
for logging. However, it spews the kernel log because of the
device name being longer. Logging with device name is also not
really needed. Move to using pr_*() variants. Add a pr_fmt() to
help that by adding a function name which will be more useful.

Change-Id: I42503ccd2b2dcc62c5c868132d202b9698c9d216
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>