spi: stm32-qspi: Fix W=1 build warning

Fix the following compilation warning using W=1 build:
arm-linux-gnueabi-ld: drivers/spi/spi-stm32-qspi.o: in function `stm32_qspi_poll_status':

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Link: https://lore.kernel.org/r/20210604075009.25914-1-patrice.chotard@foss.st.com
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/Kconfig

@@ -806,6 +806,7 @@ config SPI_STM32_QSPI
 	tristate "STMicroelectronics STM32 QUAD SPI controller"
 	depends on ARCH_STM32 || COMPILE_TEST
 	depends on OF
+	depends on SPI_MEM
 	help
 	  This enables support for the Quad SPI controller in master mode.
 	  This driver does not support generic SPI. The implementation only