ath5k: cleanup channel to eprom_mode function

Stop returning negative values from ath5k_eeprom_mode_from_channel.
Yell loudly about that case in that function instead and return the
default/zero/mode A. This cleans up the callers, but needs to pass ah
down to ath5k_eeprom_mode_from_channel for ATH5K_WARN. For that
purpose we also need the declaration to be moved to ath5k.h.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>