leds: leds-qpnp: use a single global mutex for flash led

Add a single global mutex for all instances of flash leds -
flash_0,flash_1 and flash_torch to avoid concurrency issues.

CRs-Fixed: 640712
Change-Id: I38a30432a86e47e11f975358c71557da1f58667e
Signed-off-by: Abinaya P <abinayap@codeaurora.org>