umodem(4): Clear stall at every open.

Some controllers like the XHCI(4) loose track of the data toggle value when
USB receive transfers are cancelled at close. This in turn can lead to to
data loss after the next open.

To avoid data loss, make sure both the receive and transmit data toggles
get reset, before trying to read or write any data.

Differential Revision: https://reviews.freebsd.org/D36391
Submitted by: Dave Baukus <daveb@spectralogic.com>
MFC after: 1 week
Sponsored by: NVIDIA Networking