ENGR00255406 net: fec: Workaround tx hang due to TDAR bit cleared by uDMA

ENGR00255406 net: fec: Workaround tx hang due to TDAR bit cleared by uDMA

MTIP enet IP have one IC issue recorded at PDM ticket:TKT168103

The issue description:
The TDAR bit after being set by software is not acted upon by the ENET
module due to the timing of when the ENET state machine clearing the
TDAR bit occurring coincident or momentarily after the software sets
the bit.

The result:
The corresponding transmit packet for an incoming ping is delayed.

Workaround:
This forces the ENET module to check the Transmit buffer descriptor
and take action if the “ready” flag is set. Otherwise the ENET module
returns to idle mode.

Signed-off-by: Fugang Duan  <B38611@freescale.com>