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>