mtd: spi-nor: cadence-quadspi: Fix cqspi_command_read() definition

n_tx was never used, drop it. Replace 'const u8 *txbuf' with 'u8 opcode',
to comply with the SPI NOR int (*read_reg)() method. The 'const'
qualifier has no meaning for parameters passed by value, drop it.
Going furher, the opcode was passed to cqspi_calc_rdreg() and never used,
drop it.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>