MIPS: MSI: Update MSI handling for XLP

The per-cpu interrupt ACK using EIRR has to be done just once after
all the bits in the status register are processed.

PIC ack has to be done once in case of MSI, and for every interrupt
in case of MSI-X

Signed-off-by: Jayachandran C <jchandra@broadcom.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/8887/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>