In commit 7a908832 ("spi: imx: add fallback feature") the last
user of the struct spi_imx_devtype_data::disable_dma callback was
removed. However the disable_dma member of struct spi_imx_devtype_data
and the callback itself was not removed.

Remove struct spi_imx_devtype_data::disable_dma and mx51_disable_dma()
as they are unused.

Cc: Robin Gong <yibin.gong@nxp.com>
Signed-off-by: NMarc Kleine-Budde <mkl@pengutronix.de>
Link: https://lore.kernel.org/r/20221021131051.1777984-1-mkl@pengutronix.deSigned-off-by: NMark Brown <broonie@kernel.org>