spi-cadence-quadspi: remove unused variable

Created by: MichaIng

Introduced with the last commit https://github.com/starfive-tech/linux/commit/16d2a17eb3, the kernel build now seems to correctly indicate that *cqspi is unused and hence the declaration should be removed from this function:

  CC      drivers/spi/spi-cadence-quadspi.o
drivers/spi/spi-cadence-quadspi.c: In function ‘cqspi_exec_mem_op’:
drivers/spi/spi-cadence-quadspi.c:1233:26: warning: unused variable ‘cqspi’ [-Wunused-variable]
 1233 |         struct cqspi_st *cqspi = spi_master_get_devdata(mem->spi->master);
      |