spi: imx: remove unnecessary check in spi_imx_can_dma

The spi_imx_can_dma function computes the watermark level so that
the transfer will fit in exactly N bursts (without a remainder).

The smallest watermark level possible being one FIFO entry per burst, we
can't never have a case where the transfer size isn't divsiible by 1.

Remove the extra check for the wml being different than 0.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: Mark Brown <broonie@kernel.org>