ath9k: fix misplaced semicolon on rate control

The patch e43419f9

:

	ath9k: downgrade assert in rc.c for invalid rate

downgraded an ASSERT to a WARN_ON() but also misplaced a
semicolon at the end of the second check. What this did
was force the rate control code to always return the rate
even if we should have warned about it. Since this should
not have happened anymore anyway this fix isn't critical
as the proper rate would have been returned anyway.

Cc: stable@kernel.org
Reported-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>