ath5k: fix buffer overrun in rate debug code

char bname[5] is too small for the string "X GHz" when the null
terminator is taken into account.  Thus, turning on rate debugging
can crash unless we have lucky stack alignment.

Cc: stable@kernel.org
Reported-by: Paride Legovini <legovini@spiro.fisica.unipd.it>
Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>