leds: qpnp-flash: check power supply variable in flash led driver

Check the power supply variable if it is NULL or not, in flash led
driver while enabling the flash led.
This change is added to avoid crash in flash led driver.

CRs-Fixed: 944997
Change-Id: Ide1a9151d2a7c9a6686268a53ec9e38a4b087808
Signed-off-by: Shantanu Jain <shjain@codeaurora.org>