net: fec: fix potential issue to avoid fec interrupt lost and crc error

The current flow: Set TX BD ready, and then set "INT" and "PINS" bit to
enable tx interrupt generation and crc checksum.

There has potential issue like as:
CPU			fec uDMA
Set tx ready bit
			uDMA start the BD transmission
Set "INT" bit
Set "PINS" bit
...

Above situation cause fec tx interrupt lost and fec MAC don't do
CRC checksum. The patch fix the potential issue.

Signed-off-by: Fugang Duan <B38611@freescale.com>
Acked-by: Frank Li <Frank.li@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>