mhi: core: Skip handling MSI0 if MHI register access is not allowed

Driver continues handling of MSI0 interrupt even if MHI register access
is not allowed. By doing so it calls the status call back and performs
early notification for the MHI client. This is not needed when MHI register
access is not allowed. Hence skip the handling in this case and return.

Change-Id: I4639cedfd45166f28379539bccb25acd383faca5
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>