spi/xilinx: Simplify data read from the Rx FIFO

The number of words in the read buffer will be exactly the same as the
number of words written on write buffer, once the transaction has
finished.

Instead of cheking the rx_empty flags for every word simply save the
number of words written by fill_tx_fifo.

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>