media: uvc: to the right check at uvc_ioctl_enum_framesizes()

While the logic there is correct, it tricks both humans and
machines, a the check if "i" var is not zero is actually to
validate if the "frames" var was initialized when the loop
ran for the first time.

That produces the following warning:
	drivers/media/usb/uvc/uvc_v4l2.c:1192 uvc_ioctl_enum_framesizes() error: potentially dereferencing uninitialized 'frame'.

Change the logic to do the right test instead.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>