ehci: fix frame timer invocation.

ehci registers ehci_frame_timer as both timer and bottom half, which
turned out to be a bad idea as it can be called as bottom half then
while it is running as timer, and it isn't prepared to handle recursive
calls.

Change the timer func to just schedule the bottom half to avoid this.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1449609
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20170519120428.25981-1-kraxel@redhat.com
