ath5k: ath5k_eeprom_mode_from_channel() returns signed

ath5k_eeprom_mode_from_channel() returns -1 on error but we're storing
the result in "ee_mode" which is an unsigned char.  This breaks the
error handling.  This patch makes "ee_mode" an int.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>