buswidth and dtr fields in spi_mem_op are only valid when the
corresponding spi_mem_op phase has a non-zero length. For example,
SPI NAND core doesn't set buswidth when using SPI_MEM_OP_NO_ADDR
phase.

Fix the dtr checks in set_protocol() and suppports_mem_op() to
ignore empty spi_mem_op phases, as checking for dtr field in
empty phase will result in false negatives.
Signed-off-by: Apurva Nandan <a-nandan@ti.com>
Link: https://lore.kernel.org/r/20210716232504.182-3-a-nandan@ti.comSigned-off-by: Mark Brown <broonie@kernel.org>