brcm80211: fmac: remove function brcmf_bus_get_device

brcmf_bus_get_device is no longer necessary. Use dongle device pointer
saved in brcmf_pub directly. This is part of the fullmac bus interface
refactoring.

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: John W. Linville <linville@tuxdriver.com>