spi: spi-xilinx: cleanup a check in xilinx_spi_txrx_bufs()

'!' has higher precedence than comparisons so the original condition
is equivalent to "if (xspi->remaining_bytes == 0)".  This makes the
static checkers complain.

xspi->remaining_bytes is signed and from looking at the code
briefly, I think it might be able to go negative.  I suspect that
going negative may cause a bug, but I don't have the hardware and
can't test.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mark Brown <broonie@linaro.org>