net: fec_mxc: Poll FEC_TBD_READY after polling TDAR

When testing the FEC driver on a mx6solox we noticed that the TDAR bit gets
always cleared prior then the READY bit is cleared in the last BD, which causes
FEC packets reception to always fail.

As explained by Ye Li:

"The TDAR bit is cleared when the descriptors are all out from TX ring, but on
mx6solox we noticed that the READY bit is still not cleared right after TDAR.
These are two distinct signals, and in IC simulation, we found that TDAR always
gets cleared prior than the READY bit of last BD becomes cleared.
In mx6solox, we use a later version of FEC IP. It looks like that this
intrinsic behaviour of TDAR bit has changed in this newer FEC version."

Fix this by polling the READY bit of BD after the TDAR polling, which covers the
mx6solox case and does not harm the other SoCs.

No performance drop has been noticed with this patch applied when testing TFTP
transfers on several boards of different i.mx SoCs.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Marek Vasut <marex@denx.de>