backlight: qcom-spmi-wled: Add qpnp_wled_flash_led_prepare() API

Similar to qpnp_flash_led_prepare() API, add an API for WLED
flash LED devices, qpnp_wled_flash_led_prepare() that can provide
the maximum current for flash/torch devices and maximum available
current for switch device supported by the peripheral. Camera
flash driver makes use of this to do certain operations before it
sends the request to trigger flash/torch.

Change-Id: Ieb8ed119f3d617694bf97c5ea71e1c3dac9baea1
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>