spi: imx: correct wml as the last sg length

Correct wml as the last rx sg length instead of the whole transfer
length. Otherwise, mtd_stresstest will be failed as below:

insmod mtd_stresstest.ko dev=0
=================================================
mtd_stresstest: MTD device: 0
mtd_stresstest: not NAND flash, assume page size is 512 bytes.
mtd_stresstest: MTD device size 4194304, eraseblock size 65536, page size 512, count of eraseblocks 64, pa0
mtd_stresstest: doing operations
mtd_stresstest: 0 operations done
mtd_test: mtd_read from 1ff532, size 880
mtd_test: mtd_read from 20c267, size 64998
spi_master spi0: I/O Error in DMA RX
m25p80 spi0.0: SPI transfer failed: -110
spi_master spi0: failed to transfer one message from queue
mtd_test: error: read failed at 0x20c267
mtd_stresstest: error -110 occurred
=================================================
insmod: ERROR: could not insert module mtd_stresstest.ko: Connection timed out

Signed-off-by: Robin Gong <yibin.gong@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>