ath6kl: Don't print error message when recv is canceled

An error message ath6kl_htc_rxmsg_pending_handler isn't appropate for when
the error is ECANCELED. This could be the result of a perfectly appropriate
RX cancel due to shutdown or suspend. This allows the right cleanup to
continue, but without an alarming error message in this particular case.

Signed-off-by: Steve deRosier <steve.derosier@lairdtech.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>