brcmfmac: postpone interrupt register function

For out of band interrupt which is going to be introduced shortly,
the interrupt register function must be called after firmware is
downloaded. This patch moves it from brcmf_sdbrcm_probe to
brcmf_sdbrcm_bus_init.

Reviewed-by: pieter-paul giesberts <pieterpg@broadcom.com>
Reviewed-by: arend van spriel <arend@broadcom.com>
Signed-off-by: franky lin <frankyl@broadcom.com>
Signed-off-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>