libertas: fix data packet size errors

I wondered about junk bytes at the end when using "lbsdebug +hex +host"
until I noticed that firmware for the CF card sends my extranous bytes.
It says "I have 20 bytes", I take 20 bytes, but the last 8 bytes of this
are just data junk.

Also, in the new lbs_cmd() where was a size miscalulation
that made itself clear after fixing this bug.

Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>