drivers/usb/gadget/core.c: Fix GCC 4.6 warning

Fix:
core.c: In function 'usbd_device_event_irq':
core.c:596:21: warning: variable 'state' set but not used
[-Wunused-but-set-variable]

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Remy Bohmer <linux@bohmer.net>
Acked-by: Remy Bohmer <linux@bohmer.net>
Acked-by: Marek Vasut <marek.vasut@gmail.com>