mwl8k: Set packet timestamp to 0 when life time expiry is not used

Set tx packet timestamp to 0 in following scenarios:-
- All packets in STA mode
- Mgmt packets in AP mode
- Eapol packets in AP mode

In STA mode, this field is unused in the firmware. In AP
mode, we should not be expiring mgmt and eapol frames.
Setting timestamp to 0 will ensure that.

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