[media] em28xx: use atomic bit operations for devices-in-use mask

Use atomic bit operations for the em28xx_devused mask, to prevent an
unlikely race condition should two adapters be plugged in
simultaneously. The operations also clearer than explicit bit
manipulation anyway.

Signed-off-by: Chris Rankin <rankincj@yahoo.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>