USB: pd: Drop ATTENTION command if no SVID handler is discovered

If DP adaptor sends ATTENTION command before USB PD driver sends
DISCOVER_IDENTITY command, TX command is being aborted and freed.
Due to this, DP adaptor is not getting detected as vdm state
machine is stuck at VDM_NONE. Fix it by dropping ATTENTION command
in this case unless atleast one SVID handler is discovered/connected.

Change-Id: Ib51911790f703c3497643cc99079ab20709390bc
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>