bcma: return the mips irq number in bcma_core_irq

The irq signal numbers that are send by the cpu are increased by 2 from
the number programmed into the mips core by bcma.
Return the irq number on which the irqs are send in bcma_core_irq() now.

Signed-off-by: Nathan Hintz <nlhintz@hotmail.com>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>