bcma: fix bcm4716/bcm4748 i2s irqflag

The default irqflag assignment for the I2S core on some Broadcom
4716/4748 devices is invalid and needs to be corrected (from the
Broadcom SDK).

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