smsc95xx: Use zero length packets when RX fifo is empty

Using NAKs on empty RX fifo for bulk in transfers is the right choice
for a interrupt driven model, but U-Boot uses polling and expects an
immediate answer if there is no incoming packet. Using ZLP Bulk In Response
(BIR) mode avoids unexpected timeouts in the host controller driver.

As ZLP mode is reset default, there is no need to set it.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>