leds: leds-qpnp: check if flash_wa regulator is obtained before enabling it

Enable flash_wa regulator in qpnp_flash_regulatori_operate function only if
it is obtained in the first place.

Change-Id: Ibddda5327a792edecfe61fca451dbd0bfff2bda2
Signed-off-by: Abinaya P <abinayap@codeaurora.org>