staging: brcm80211: remove function pointer of interrupt isr in brcmfmac

The use of function pointer of bus interrupt isr is no longer needed
in fullmac as there is only one available isr.

Reviewed-by: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>