USB: Clear "warm" port reset change.

In USB 3.0, there are two types of resets: a "hot" port reset and a "warm"
port reset.  The hot port reset is always tried first, and involves
sending the reset signaling for a shorter amount of time.  But sometimes
devices don't respond to the hot reset, and a "Bigger Hammer" is needed.

External hubs and roothubs will automatically try a warm reset when the
hot reset fails, and they will set a status change bit to indicate when
there is a "BH reset" change.  Make sure the USB core clears that port
status change bit, or we'll get lots of status change notifications on the
interrupt endpoint of the USB 3.0 hub.

(Side note: you may be confused why the USB 3.0 spec calls the same type
of reset "warm reset" in some places and "BH reset" in other places.  "BH"
reset is supposed to stand for "Big Hammer" reset, but it also stands for
"Brad Hosler".  Brad died shortly after the USB 3.0 bus specification was
started, and they decided to name the reset after him.  The suggestion was
made shortly before the spec was finalized, so the wording is a bit
inconsistent.)

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>

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