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

[ Upstream commit 6d6af579

 ]

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: c5c31fb7

 ("spi: fsl-dspi: avoid SCK glitches with continuous transfers")
Signed-off-by: Sasha Levin <sashal@kernel.org>