powerpc/mpic: Fix confusion between hw_irq and virq

mpic_is_ipi() takes a virq and immediately converts it to a hw_irq.

However, one of the two call sites calls it with a ... hw_irq. The
other call site also happens to have the hw_irq at hand, so let's
change it to just take that as an argument. Also change mpic_is_tm()
for consistency.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>