[MIPS] PNX8550 fixups

This patch fixes the compilation errors on PNX8550 and hard-to-track
bug in interrupt handling.
It also corresponds to the latest changes in PNX8550 serial driver.

Signed-off-by: Vitaly Wool <vitalywool@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>