Make use of a core helper to ensure the desired width is respected
when calling spi-mem operators.

Otherwise only the SPI controller will be matched with the flash chip,
which might lead to wrong widths. Also consider the width specified by
the user in the device tree.

Fixes: a5356aef ("spi: spi-mem: Add driver for NXP FlexSPI controller")
Signed-off-by: Michael Walle <michael@walle.cc>
Link: https://lore.kernel.org/r/20191211195730.26794-1-michael@walle.ccSigned-off-by: Mark Brown <broonie@kernel.org>