spi: Increase imx51 ecspi burst length based on transfer length

IMX51 supports 4096 bit burst lengths. Using the spi transfer length
instead of bits_per_word increases performance significantly.

Signed-off-by: Stefan Moring <stefan.moring@technolution.nl>
Link: https://lore.kernel.org/r/20230628125406.237949-1-stefan.moring@technolution.nl


Signed-off-by: Mark Brown <broonie@kernel.org>