ARM: at91: Fix USB AT91 gadget registration

Since 193ab2a6

, various AT91 boards don't
register at91_udc anymore due to depending on a now non-existing symbol.
Fix the symbol name.

Signed-off-by: Anders Darander <anders@chargestorm.se>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: stable <stable@vger.kernel.org>