Microblaze implement enable/disable interrupts through MSR
that's why disable_interrupts function should return 1 when interrupt
was enabled. Return 0 when interrupt was disabled.

Signed-off-by: John Linn <john.linn@xilinx.com>
Signed-off-by: Michal Simek <monstr@monstr.eu>
Acked-by: Wolfgang Denk <wd@denx.de>