brcmsmac: remove support for cc rev < 20

The lowest chip common version used on bcma based devices is 31 on the
bcm4718 and 32 on the bcm4313, bcm43224, and bcm43225, so the support
for the old versions could be removed.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Tested-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>