mwifiex: Command 7 handling for USB chipsets

Firmware image for newer USB chipsets starts with a command 7 block
(special command). It doesn't contain data length field. This patch adds
necessary handling.

Signed-off-by: Ganapathi Bhat <gbhat@marvell.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>