V4L/DVB: V4L: dvb-usb, add extra sync to down-up input events

Userspace is allowed to coalesce events between SYNCs. And since the code
emits UP right after DOWN for the same key, it may be missed
(up+down=nothing). Add an extra sync in between UP and DOWN events to disable
the coalesce.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Patrick Boettcher <pboettcher@kernellabs.com>
Acked-by: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>