brcm80211: fmac: register a dummy irq handler for SDIO function 2

When there is data available in dongle for driver to fetch, an
interrupt will be triggered and both SDIO function 1 and 2 will be
flagged by default in bcm4329. These two interrupt flags are
identical and only need to be handled once. Since there is no way
to turn off one flag from the dongle side, a dummy handler for
function 2 interrupt is placed.

Reported-by: Denis 'GNUtoo' Carikli <GNUtoo@no-log.org>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Arend van Spriel <arend@broadcom.com>
Reviewed-by: Alwin Beukers <alwin@broadcom.com>
Signed-off-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>