libertas_tf: don't set URB_ZERO_PACKET on IN USB transfer

It doesn't make sense and the USB core warns on each submit of such
URB, easily flooding the message buffer with tracebacks.

Analogous issue was fixed in regular libertas driver in commit 6528d880
("libertas: don't set URB_ZERO_PACKET on IN USB transfer").

Cc: stable@vger.kernel.org
Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Reviewed-by: Steve deRosier <derosier@cal-sierra.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>