bcma: fix null pointer in bcma_core_pci_irq_ctl

pc could be null if hosttype != BCMA_HOSTTYPE_PCI.
If we are on a device without a pci core this function is called with
pc = null by b43 and brcmsmac. If the host type is PCI we have a pci
core as well and pc can not be null.

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