drivers/linuxspidev: fix minor memory leak

free() strduped spi_path on quit.
Found by valgrind.

Change-Id: Iaa59c7258c920b5e60d615df790dfe815831b925
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Fixes: 83e0293f7b ("Add Linux SPI device SWD adapter support")
Reviewed-on: https://review.openocd.org/c/openocd/+/8732
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Richard Pasek <rpasek@google.com>

src/jtag/drivers/linuxspidev.c

@@ -337,6 +337,10 @@ static int spidev_quit(void)
 
 	close(spi_fd);
 	spi_fd = -1;
+
+	free(spi_path);
+	spi_path = NULL;
+
 	return ERROR_OK;
 }