spi: dw: Fix broken dw_spi_mem_ops()

The driver is currently using sizeof(op->cmd.opcode) in the op_len
calculation. Commit d15de623 ("spi: spi-mem: allow specifying a
command's extension") changed op->cmd.opcode from one byte to two.

Instead, a new struct member op->cmd.nbytes is supposed to be used.
For regular commands op->cmd.nbytes will be one.

Commit d15de623 ("spi: spi-mem: allow specifying a command's
extension") did update some drivers that overload the generic mem_ops()
implementation, but forgot to update dw_spi_mem_ops().

Calculating op_len incorrectly causes dw_spi_mem_ops() to misbehave, since
op_len is used to determine how many bytes that should be read/written.

On the canaan k210 board, this causes the probe of the SPI flash to fail.

Fix the op_len calculation in dw_spi_mem_ops(). Doing so results in
working SPI flash on the canaan k210 board.

Fixes: d15de623 ("spi: spi-mem: allow specifying a command's extension")
Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
Reviewed-by: Pratyush Yadav <p.yadav@ti.com>
Tested-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>