usb: mtu3: cleanup with list_first_entry_or_null()

The combo of list_empty() and list_first_entry() can be replaced with
list_first_entry_or_null().

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>