net: ethernet: ti: prueth: fix handling packets with timestamp

The packets with timestamp (TS) got the TS placed in the data block
following the packet data and TS data block has corresponding Buffer
descriptor allocated. Now ICSS prueth driver reads packet data and TS
properly, but releases only Buffer descriptors allocated for Packet data
and misses Buffer descriptor allocated for TS. As result, the queue rd_ptr
points on Buffer descriptor allocated for TS when driver switches to next
packet processing which is all 0.

This situation is detected by driver as critical error and driver tries to
recover by discarding all existing packets in the RX queue (rd_ptr =
wr_ptr).

Hence, fix packets with timestamp processing by properly account Buffer
descriptor allocated for TS.

Fixes: 813f7dc8

 ("net: ti: prueth: Add support for reading timestamps from rx packets")
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>