soc: swr-mstr: reset slave pending irq flag before every call

Before every slave interrupt handler call, reset slave pending
irq flag, so that if irq is halted it does not retain the old
value.

Change-Id: Ibde5dacf38e78ab19afb6f0e91ffac81f6c22813
Signed-off-by: Ramprasad Katkam <katkam@codeaurora.org>