Do not skip interrupt sources in sun4d interrupt handler and acknowledge interrupts correctly

During the introduction of genirq on sparc32 bugs were introduced in
the interrupt handler for sun4d. The interrupts handler checks the status
of the various sbus interfaces in the system and generates a virtual
interrupt, based upon the location of the interrupt source. This lookup
was broken by restructuring the code in such a way that index and shift
operations were performed prior to comparing this against the values
read from the interrupt controllers.

This could cause the handler to loop eternally as the interrupt source
could be skipped before any check was performed. Additionally
sun4d_encode_irq performs shifting internally, so it should not be performed
twice.

In sun4d_unmask interrupts were not correctly acknowledged, as the
corresponding bit it the interrupt mask was not actually cleared.

Signed-off-by: Kjetil Oftedal <oftedal@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>