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: I2c78500e83eca7864029f58cffa80f8648902fb2
Signed-off-by: Meng Wang <mengw@codeaurora.org>