net: macb: Add small delay after link establishment

I've noticed that the first ethernet packet after PHY link establishment
is not tranferred correctly most of the time on my AT91SAM9G25 board.
Here I usually see a timeout of a few seconds, which is quite
annoying.

Adding a small delay (10ms in this case) after the link establishment
helps to solve this problem. With this patch applied, this timeout
on the first packet is not seen any more.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Wenyou Yang <wenyou.yang@atmel.com>
Cc: Eugen Hristev <eugen.hristev@microchip.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>