spi: atmel-quadspi: Fix the buswidth adjustment between spi-mem and controller

Use the spi_mem_default_supports_op() core helper in order to take into
account the buswidth specified by the user in device tree.

Fixes: 24c8ff46 ("spi: Add Atmel QuadSPI driver")
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>