MIPS: Lantiq: Use the real EXIN count

MIPS: Lantiq: Use the real EXIN count

We runtime load the available external interrupts into an array and store
the number inside exin_avail. Some of the code however uses MAX_EIU for
looping over the array which may partially be 0. This is a cosmetic fix as
the existing code works as is. It is just nicer to only loop over the array
elements that were actually populated during probe.

Signed-off-by: John Crispin <john@phrozen.org>
Cc: Linux-MIPS <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/13602/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>