ath5k: fix hw rate index condition

Make sure we print out a warning when the index is out of bounds,
i.e. even on hw_rix == AR5K_MAX_RATES.

Also change to WARN and print text with the reported hw_rix.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Cc: Nick Kossifidis <mickflemm@gmail.com>
Cc: Luis R. Rodriguez <lrodriguez@atheros.com>
Cc: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>