leds: qpnp-flash: Introduce a function pointer for qpnp_flash_led_prepare

Introduce a function pointer to avoid the compilation error
due the same function name used in the legacy and v2 flash
drivers. These two drivers need to be enabled in a single
defconfig that supports multiple hardware platforms to achieve
the binary compatibility.

Change-Id: I1ded3d4d9013c6f7f56e0a8952f4b45d20b07326
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>