leds-qpnp-flash: Add a stub function for qpnp_flash_led_prepare()

When CONFIG_LEDS_QPNP_FLASH_V2 option is unselected, drivers that
make use of qpnp_flash_led_prepare() fails the compilation as the
reference is not available. Add a stub function to allow them
compile.

Change-Id: I1ae8457f6aaa655dfbbc63677df6c2f6416ad029
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>