ath5k: remove most references to XR

XR is a proprietary feature of the chipset.  It's not supported and
should not be supported.

Signed-off-by: Pavel Roskin <proski@gnu.org>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>