MIPS: OCTEON: Remove some code limiting NR_IRQS to 255

MIPS: OCTEON: Remove some code limiting NR_IRQS to 255

Follow-on patches for OCTEON III will increase the number of irqs to
potentially more than 256.

Increase the width of the octeon_irq_ciu_to_irq to int to be able to
handle this case.  Remove the hacky code that verified that u8 would
not be overflowed.

Signed-off-by: David Daney <david.daney@cavium.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/12495/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>