libertas: rename and re-type bufvirtualaddr to cmdbuf

Make it a struct cmd_header, since that's what it is, and clean up
the places that it's used.

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