leds: qpnp-flash: Add support to query maximum current

Currently, MAX_CURRENT option is used to query the maximum
available current which is a relative value based on the battery
parameters. Rename it to MAX_AVAIL_CURRENT so that MAX_CURRENT
option can be used to query the maximum current that is supported
for flash LED devices.

Change-Id: Id48327c818a42e4407627055c6b243591ab441fe
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>