powerpc/mpic: Remove MPIC_BROKEN_FRR_NIRQS and duplicate irq_count

The mpic->irq_count variable is only used as a software error-checking
limit to determine whether or not an IRQ number is valid.  In board code
which does not manually specify an IRQ count to mpic_alloc(), i.e. 0, it
is automatically detected from the number of ISUs and the ISU size.

In practice, all hardware ends up with irq_count == num_sources, so all
of the runtime checks on mpic->irq_count should just check the value of
mpic->num_sources instead.

When platform hardware does not correctly report the number of IRQs,
which only happens on the MPC85xx/MPC86xx, the MPIC_BROKEN_FRR_NIRQS
flag is used to override the detected value of num_sources with the
manual irq_count parameter.  Since there's no need to manually specify
the number of IRQs except in this case, the extra flag can be eliminated
and the test changed to "irq_count != 0".

Signed-off-by: Kyle Moffett <Kyle.D.Moffett@boeing.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>