brcmfmac: remove brcmf_sdbrcm_wait_for_event

brcmf_sdbrcm_wait_for_event is now a one line function and only
used by brcmf_sdbrcm_bus_txctl. Intergrate the function call
wait_event_interruptible_timeout into brcmf_sdbrcm_bus_txctl.

Signed-off-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>