libertas: kill lbs_pre_start_xmit(), lib_mesh_pre_start_xmit()

These wrappers only do two things.

Firstly, they set the frame type, which isn't necessary since
lbs_hard_start_xmit() gets to see which device it belongs to anyway.

Secondly, they return -EOPNOTSUPP if the device is in monitor mode.
Which is a strange thing to do and will provide nasty warnings from
qdisc_restart(). And lbs_hard_start_xmit() seems to have code to cope
with monitor mode anyway.

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