leds: qpnp-flash: Implement the flash_prepare API

Add the flash_prepare API to the leds-qpnp-flash driver to
allow camera driver to query the maximum available flash current.

CRs-Fixed: 1043718
Change-Id: I21cc4b4556b2a4a1a918bfeb1d55ae164d4b5568
Signed-off-by: Devesh Jhunjhunwala <deveshj@codeaurora.org>