spi: cadence-quadspi: Use spi_mem_dtr_supports_op()

Use the newly introduced spi_mem_dtr_supports_op() to check DTR op
support. This means the buswidth check does not need to be replicated.
It also happens to fix a bug where STR ops with a 2-byte opcode would be
reported as supported.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Link: https://lore.kernel.org/r/20210204141218.32229-2-p.yadav@ti.com
Signed-off-by: Mark Brown <broonie@kernel.org>