[MIPS] msp_hwbutton.c: minor irq handler cleanups

- remove always-true test

- neaten request_irq() indentation

This change's main purpose is to prepare for the patchset in
jgarzik/misc-2.6.git#irq-remove, that explores removal of the
never-used 'irq' argument in each interrupt handler.

Signed-off-by: Jeff Garzik <jgarzik@redhat.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>