libertas: remove "struct cmd_ds_gen"

It was only used as a source for S_DS_GEN, but the size of this struct
is equal to the size of "struct cmd_header".

Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>