iwl4965: correctly validate temperature value

In some cases we can read wrong temperature value. If after that
temperature value will not be updated to good one, we badly configure
tx power parameters and device is unable to send a data.

Resolves:
https://bugzilla.kernel.org/show_bug.cgi?id=35932

Cc: stable@kernel.org # 2.6.39+
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>