orinoco: refactor xmit path

... so orinoco_usb can share some common functionality.

Handle 802.2 encapsulation and MIC calculation in that function.
The 802.3 header is prepended to the SKB. The calculated MIC is written
to a specified buffer. Also modify the transmit control word that will
be passed onto the hardware to specify whether the MIC is present, and
the key used.

Signed-off-by: David Kilroy <kilroyd@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>