NET: ath5k, check ath5k_eeprom_mode_from_channel retval

It can, if invalid argument given, return a negative value. In that
case we would access arrays out-of-bounds and such. Check the value
and yell loudly if that happened as it would be a bug in the
implementation. (Instead of silently corrupting memory.)

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Nick Kossifidis <mickflemm@gmail.com>
Cc: "Luis R. Rodriguez" <mcgrof@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>