msm: ipa: add interface for restoring IPA suspend handler

Other drivers are able to remove existing suspend IRQ handler.
This change enables to restore the original suspend handler
which is registered on IPA initialization.

Change-Id: I88eec1c21b02424b1c09ff622d796b3e8d260931
Signed-off-by: Amir Levy <alevy@codeaurora.org>