libertas: make lbs_cmd() usage nicer

Define a macro that relieves the caller from having to use sizeof on
the command structure when calling lbs_cmd(), and move the prototype
of __lbs_cmd() to a new cmd.h file.

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>