usb: gadget: r8a66597: Add missing null check on return from platform_get_resource

usb: gadget: r8a66597: Add missing null check on return from platform_get_resource

The call to platform_get_resource can potentially return a NULL pointer
on failure, so add this check and return -EINVAL if it fails.

Fixes: 20bdf4542695 ("usb: gadget: R8A66597 peripheral controller support.")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Addresses-Coverity: ("Dereference null return")
Link: https://lore.kernel.org/r/20210406184510.433497-1-colin.king@canonical.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>