brcmsmac: select CONFIG_BCMA when possible

The brcmsmac relies BCMA functionality to access the device. This
patch selects CONFIG_BCMA when CONFIG_BCMA_POSSIBLE is set. This
way the user does not need to be select BCMA to make the brcmsmac
driver show up in his menuconfig.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>