NFC: delete null dereference

The exit label performs device_unlock(&dev->dev);, which will fail when dev
is NULL, and nfc_put_device(dev);, which is not useful when dev is NULL, so
just exit the function immediately.

Problem found using scripts/coccinelle/null/deref_null.cocci

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>