ath5k: check return value of ieee80211_get_tx_rate

ath5k: check return value of ieee80211_get_tx_rate

commit d8e1ba76d619dbc0be8fbeee4e6c683b5c812d3a upstream.

This avoids a NULL pointer dereference as reported here:

	https://bugzilla.redhat.com/show_bug.cgi?id=625889

When the WARN condition is hit in ieee80211_get_tx_rate, it will return
NULL.  So, we need to check the return value and avoid dereferencing it
in that case.

Signed-off-by: John W. Linville <linville@tuxdriver.com>
Acked-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>