bcma: add gpio_to_irq

The old bcm47xx gpio code had support for gpio_to_irq, but the new
code did not provide this function, but returned -ENXIO all the time.
This patch adds the missing function.

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