drivers: bcma: export bcma_core_disable() function

In the brcm80211 driver we disable the 80211 core when the driver is
'down'. The bcma_core_disable() function exactly does the same as
our implementation so exporting this function makes sense.

Cc: linux-wireless@vger.kernel.org
Cc: Rafal Milecki <zajec5@gmail.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>