ums(4): Do not stop USB xfers on FIFO close when evdev is still active

This fixes lose of evdev events after moused has been killed.

While here use bitwise operations for UMS_EVDEV_OPENED flag.

Reviewed by: hselasky
Differential revision: https://reviews.freebsd.org/D30342

(cherry picked from commit 05ab03a31798d4cc96c22a8f30b1d9a0d7a3dd35)