spi: cadence-quadspi: Fix check condition for DTR ops

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() 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>
Signed-off-by: Dhruva Gole <d-gole@ti.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>