libertas: handy function to call firmware commands

Using an arbitrary firmware command was actually very painful. One
had to change big switch() statements in cmd.c, cmdresp.c, add
structs to the big union in "struct cmd_ds_command" and add the
define for the CMD_802_11_xxx to the proper place.

With this function, this is now much easier. For now, it implements
a blocking (a.k.a. CMD_OPTION_WAITFORRSP) way where one deals directly
with command requests and response buffers. You can do everything in
one place:

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>