carl9170: fix carl9170_tx_prepare typo

commit: "carl9170: revamp carl9170_tx_prepare"
introduced a peculiar bug that would only show
up if the the module parameter noht is set to 1.

Then all outbound voice, video and background
frames would each invoke a (bogus) RTS/CTS
handshake.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>