brcmsmac: document firmware dependencies

The brcmsmac driver requests firmware but doesn't document the
dependency. This means that software that analyzes the modules to
determine if firmware is needed won't detect it.

Specifically, (at least) openSUSE won't install the kernel-firmware
package if no hardware requires it.

This patch adds the MODULE_FIRMWARE directives.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Acked-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>