ath9k: Do not assign noise for NULL caldata

ah->noise is maintained globally and not per-channel. This
is updated in the reset() routine after the NF history has been
filled for the *current channel*, just before switching to
the new channel. There is no need to do it inside getnf(), since
ah->noise must contain a value for the new channel.

Cc: stable@vger.kernel.org
Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>