staging: olpc_dcon_xo_1: add missing 'const' qualifier

gcc noticed a mismatch between the type qualifiers after a recent
cleanup:

drivers/staging/olpc_dcon/olpc_dcon_xo_1.c: In function 'dcon_init_xo_1':
drivers/staging/olpc_dcon/olpc_dcon_xo_1.c:48:26: error: initialization discards 'const' qualifier from pointer target type [-Werror=discarded-qualifiers]

Add the 'const' keyword that should have been there all along.

Fixes: 2159fb37 ("staging: olpc_dcon: olpc_dcon_xo_1.c: Switch to the gpio descriptor interface")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/olpc_dcon/olpc_dcon_xo_1.c

@@ -45,7 +45,7 @@ static int dcon_init_xo_1(struct dcon_priv *dcon)
 {
 	unsigned char lob;
 	int ret, i;
-	struct dcon_gpio *pin = &gpios_asis[0];
+	const struct dcon_gpio *pin = &gpios_asis[0];
 
 	for (i = 0; i < ARRAY_SIZE(gpios_asis); i++) {
 		gpios[i] = devm_gpiod_get(&dcon->client->dev, pin[i].name,