FEC: Rework the TX wait mechanism

The mechanism waiting for transmission to finish in fec_send() now
relies on the E-bit being cleared in the TX buffer descriptor. In
case of data cache being on, this means invalidation of data cache
above this TX buffer descriptor on each test for the E-bit being
cleared.

Apparently, there is another way to check if the transmission did
complete. This is by checking the TDAR bit in the X_DES_ACTIVE
register. Reading a register does not need any data cache invalidation,
which is beneficial.

Rework the sequence that wait for completion of the transmission so that
the TDAR bit is tested first and afterwards check the E-bit being clear.
This cuts down the number of cache invalidation calls to one.

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>