mailbox: msm_qmp: Remove IRQ clear

IRQ clear was wrongly added because an interrupt was incorrectly
configured as a level interrupt. The qmp driver should not clear this
interrupt since there is a chance the interrupt will not be read by the
remote.

Change-Id: Id68af1db0d9ac213958a5b088f1d1c80eb5395a9
Signed-off-by: Chris Lew <clew@codeaurora.org>