mwifiex: enable auto deep sleep mode for USB chipsets

Chip goes into low power state when this feature is enabled.
This was already enabled for SDIO and PCIe interface based
chipsets.

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