drivers: spi: spi_mcux_ecspi: Fix data size when using 16/32 bit transfers

The data size is set using a burst length, the data size for 8/16/32 is
always 1 in those cases.

Signed-off-by: Pieter De Gendt <pieter.degendt@basalte.be>
(cherry picked from commit 0e8aed73)