ag71xx: Move timestamp struct member outside of struct.

With this change, the timestamp variable is only used in ag71xx_check_dma_stuck. Small tx speedup.

Based on a Qualcomm commit. ag->timestamp = jiffies was not replaced with netif_trans_update(dev) because of this quote:

It should be noted that after this series several instances
of netif_trans_update() are useless (if they occur in
.ndo_start_xmit and driver doesn't set LLTX flag -- stack already
did an update).

From: http://lists.openwall.net/netdev/2016/05/03/87



Signed-off-by: Rosen Penev <rosenp@gmail.com>