mwl8k: Add timestamp information for tx packets

Timestamp tx packets using a HW micro-second timer.
This timestamp will be compared to the current timestamp
in the hardware and if the difference is greater than 500ms,
the packet will be dropped.

Signed-off-by: Pradeep Nemavat <pnemavat@marvell.com>
Signed-off-by: Nishant Sarmukadam <nishants@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>