spi: spi-fsl-dspi: Remove unused chip->void_write_data

spi: spi-fsl-dspi: Remove unused chip->void_write_data

[ Upstream commit 6d6af5796e5d9a88ae83c9c753023bba61deb18b ]

This variable has been present since the initial submission of the
driver, and held, for some reason, the value of zero, to be sent on the
wire in the case there wasn't any TX buffer for the current transfer.

Since quite a while now, however, it isn't doing anything at all.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20200304220044.11193-3-olteanv@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>
Stable-dep-of: c5c31fb71f16 ("spi: fsl-dspi: avoid SCK glitches with continuous transfers")
Signed-off-by: Sasha Levin <sashal@kernel.org>