libertas: kill lbs_upload_tx_packet()

It replaces two lines of code. And even for those it has to make
inferences about things (i.e. which device) which the caller would have
just known.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>