usb: pd: Fix Alert/Get_Status handling

When an ALERT message is received, GET_STATUS should not be sent
right away as that is not part of its atomic message sequence.
Further, as a sink, this also violates collision avoidance rules
since the ALERT is sent during SinkTxNG. Instead, queue up the
GET_STATUS to be sent when Rp is back to SinkTxOk.

In addition, replace the ado sysfs entry with get_status since the
former is not very useful to userspace. Reading from this entry
will trigger a GET_STATUS to be sent and block until the STATUS
is received.

Change-Id: I6943e1e8fab300c13ec94c516e89fc466dac8209
Signed-off-by: Jack Pham <jackp@codeaurora.org>