[nrf fromtree] drivers: nrf_qspi_nor: Fix build without multithreading

Fix build of nrf_qspi_nor flash driver without multithreading enabled.
This is required for builds like mcuboot.

Signed-off-by: Joakim Andersson <[email protected]>
(cherry picked from commit 14c896b)

Author: joerchan

drivers/flash/nrf_qspi_nor.c

@@ -487,7 +487,9 @@ static int qspi_wait_while_writing(const struct device *dev, k_timeout_t poll_pe
 	int rc;
 
 	do {
+#ifdef CONFIG_MULTITHREADING
 		k_sleep(poll_period);
+#endif
 		rc = qspi_rdsr(dev, 1);
 	} while ((rc >= 0)
 		 && ((rc & SPI_NOR_WIP_BIT) != 0U));