Remove the loop in the cascade handler and instead unconditionally
handle just the first set interrupt coming from the CPLD.

This gets rid of a lot of spurious interrupts being triggered for
the SMSC91111 ethernet chip especially under high(er) IDE load:
"eth0: spurious interrupt (mask = 0xb3)"

Verified on DB1200 and DB1300.
Signed-off-by: NManuel Lauss <manuel.lauss@googlemail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3288/Signed-off-by: NRalf Baechle <ralf@linux-mips.org>