spi: imx: Revert "spi: imx: dynamic burst length adjust for PIO mode"

This reverts commits 8d4a6cad and
179547e1.

Besides the problems already found with this patch it also modifies
the spi transfer tx_buf in spi_imx_u32_swap_u8() and spi_imx_u32_swap_u16().
This is hidden from the compiler with an explicit cast from const void*
to u32*, so no warning is issued.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>