msm: bam_dmux: change write_lock acquire mode

Change writelock() to write_trylock_irqsave().  The irqsave prevents
deadlock when an interrupt is handled while the write_lock is held, and
the irq attempts to grab the read_lock.  The trylock prevents deadlock
when ul_timeout() is scheduled while the read_lock is held thus the
write_lock cannot be grabbed and irqsave prevents scheduling.

Change-Id: I19e7c995e645d447e93559a6fbb10ff08ee80815
CRs-Fixed: 319109
Signed-off-by: Jeffrey Hugo <jhugo@codeaurora.org>