FEC: Remove endless loop in the FEC driver

The FEC hardware sometimes errors out on data transfer and hangs in
the tightloop adjusted by this patch. So add timeout into the tightloop
to make such a hang recoverable.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Otavio Salvador <otavio@ossystems.com.br>
Cc: Stefano Babic <sbabic@denx.de>