When running application tool switchtec-user's `firmware update` and `event
wait` commands concurrently, sometimes the firmware update speed reduced
significantly.

It is because when the MRPC event happened after MRPC event occurrence
check but before the event mask loop reaches its header register in event
ISR, the MRPC event would be masked unintentionally.  Since there's no
chance to enable it again except for a module reload, all the following
MRPC execution completion checks time out.

Fix this bug by skipping the mask operation for MRPC event in event ISR,
same as what we already do for LINK event.

Fixes: 52eabba5 ("switchtec: Add IOCTLs to the Switchtec driver")
Signed-off-by: Wesley Sheng <wesley.sheng@microchip.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>