serial: remove watch on reset

Otherwise, this can cause serial_xmit to be entered with LSR.TEMT=0,
which is invalid and causes an assertion failure.

Reported-by: Bret Ketchum <bcketchum@gmail.com>
Tested-by: Bret Ketchum <bcketchum@gmail.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
