MIPS: Simplify and correct interrupt handling for MSP4200

The current interrupt handling code for the MSP4200 always masks an
interrupt before acknowledging it.  This is not required, as that will be
handled by the level interrupt handler.  This change simplifies the MSP4200
code to remove the masking in the ack routine, and makes sure that the
minimum required operation is performed for masking and acking, rather
than always both masking and acking the interrupt.

Signed-off-by: Shane McDonald <mcdonald.shane@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>