xilinx_intc: Fix writes into MER register

The MER register only has two valid bits. This is now used by
the linux kernel to auto-detect endianness, and causes Linux 3.15-rc1
and later to hang when run under qemu-microblaze. Mask valid bits before
writing the register to solve the problem.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>
[Edgar: Untabified]
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>
