ath9k: Remove bogus assert in ath_clone_txbuf()

oops, this one should be part of the original patch
"ath9k: downgrade assert in ath_clone_txbuf()"

Signed-off-by: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>