spi: sun4i: fix FIFO limit

commit 6d9fe44b upstream.

When testing SPI without DMA I noticed that filling the FIFO on the
spi controller causes timeout.

Always leave room for one byte in the FIFO.

Signed-off-by: Michal Suchanek <hramrach@gmail.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>