bcma: jump to 'out' label for invalid 'func' value

Consistently jump to the 'out' label for error conditions (adds
missing check for 'func' validity in bcma_extpci_write_config).

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>