[wireless prism54] Fix frame length

prism54 is leaking information when passing transmits to the firmware.
There is no requirement to adjust the length to >= ETH_ZLEN.
Just pass the skb length (after possible adjustment).

Signed-off-by: Roger While <simrw@sim-basis.de>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>