ath5k: fix build break from "ath5k: Print out opmode in debugfs"

Also improve ath_opmode_to_string usage by having it return UNKNOWN
rather than NULL in the event of failure to map the opmode value to a
representative string.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>