The CBUS UART's interrupt number was wrong conflicting with the interrupt
being tied to the Intel PIIX4.  Since the PIIX4's interrupt is registered
before the CBUS UART which is not being used on most systems this would
not be noticed.

Attempts to open the ttyS2 CBUS UART would result in:

genirq: Flags mismatch irq 18. 00000000 (serial) vs. 00010000 (XT-PIC cascade)
serial_link_irq_chain: request failed: -16 for irq: 18

Qemu was written to match the kernel so will need to be fixed also.
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>