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

The flash led requires the use of an optional regulator, "flash_wa_reg".
This regulator is enabled without checking if the regulator is obtained
or not. Hence, a check is made before enabling/disabing the regulator to
ensure that the "flash_wa_reg" is obtained first.

Change-Id: I8fda00274ad3bc21d9ed32d0f3f2ca09e1079f87
Signed-off-by: Abinaya P <abinayap@codeaurora.org>