libertas: add simple copyback command callback

A simple callback which copies the response back into the
command buffer that was used to send the command to the
card.  Will allow for direct command processing outside
the mega-switches in cmd.c and cmdresp.c.

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>