libertas: kill (IS,SET,UNSET)_MESH_FRAME.

No need for these any more. We've collapsed all the unneeded nests of
functions which needed to keep track of which device the skb belonged to.

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