MIPS: ath79: simplify MISC IRQ handling

The current code uses multiple if statements for
demultiplexing the different interrupt sources.
Additionally, the MISC interrupt controller has
32 interrupt sources and the current code does not
handles all of them.

Get rid of the if statements and process all interrupt
sources in a loop to fix these issues.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/4874/
Signed-off-by: John Crispin <blogic@openwrt.org>