libertas: change inference about buffer size in lbs_cmd()

sizeof(*cmd) is going to give the total size of the data structure that
we allocated, more often than not. But the size of the command to be
_sent_ could be a lot smaller, as it is in the KEY_MATERIAL and
SUBSCRIBE_EVENT commands for example. So swap them round; let the caller
set the _command_ size explicitly in the header, and infer the
maximum response size from the data structure.

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