mwifiex: Support USB interrupt endpoint for command response/event

USB firmware added support for sending command response/event through
interrupt endpoint, to enhance RX throughput. Added corresponding changes
required to support this feature. This change takes care of backward
compatibility with older firmware.

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