ums(4): Start USB xfers on opening of evdev node unconditionally.

This fixes inability to start USB xfers in a case when FIFO has been
already open()-ed but no read() or poll() calls has been issued yet.

MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D30343

(cherry picked from commit 47791339f0cfe3282a6f64b5607047f7bca968ad)