leds: qti-flash: Add support for qti_flash_led_prepare()

qti_flash_led_prepare() API is used by flash clients (E.g. Camera) to
query maximum current available for flash. Add support for this API
which initially returns maximum current that is supported for flash
LED module (2000 mA). Also, add a sysfs parameter under switch devices
to show max available flash current.

Change-Id: I3aed009a0d7e0f34a6d05a45af34fcd0f37a6fe7
Signed-off-by: Shyam Kumar Thella <sthella@codeaurora.org>