ath5k: discard 11g caps if reported by an ar5211 eeprom

At least one ar5211 card (GIGABYTE GN-WLMA101, 168c:0012 subsystem
1458:e800) reports itself as 11g capable which seems to be a bug in the
eeprom. initvals.c assumes that ar5211 is only 11b capable and thus refuses
to initialize this card. Hence this patch changes the probing for 11g
capabilities to discard 11g capabilities for ar5211 cards which allows this
specific card to work fine in 11b and 11a modes.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>