spi/xilinx: Simplify spi_fill_tx_fifo

Instead of checking the TX_FULL flag for every transaction, find out the
size of the buffer at probe time and use it.

To avoid situations where the core had some data on the buffer before
initialization, the core is reseted before the buffer size is detected

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