drivers/mtd_spi_nor: fix hang

This patch releases the SPI bus when the memory devices is not found.
This frees the bus for other devices to use the bus. It also allows
future attempts to init the device.

drivers/mtd_spi_nor/mtd_spi_nor.c

@@ -458,6 +458,7 @@ static int mtd_spi_nor_power(mtd_dev_t *mtd, enum mtd_power_state power)
                 retries++;
             } while (res < 0 && retries < MTD_POWER_UP_WAIT_FOR_ID);
             if (res < 0) {
+                mtd_spi_release(dev);
                 return -EIO;
             }
             /* enable 32 bit address mode */