usb: dwc3-msm: Ensure separate EUD extcon dev is used

USB dwc3 driver is registering for VBUS, ID, and EUD based notification
for USB cable status. VBUS and ID can be notified with same edev if
board design support that, but EUD based notification needs to come by
EUD extcon managed by EUD driver. Here in absent of EUD extcon phandler
current code is using previous VBUS related edev handler and registering
EUD related notifier callback. This results into EUD related notifier
callback being called after VBUS related notifier callback although there
is no EUD extcon change. Fix this issue by explicitly marking edev to
NULL before querying for EUD extcon.

Change-Id: I9aa8406f28165ba986d693fdf8a2e328192a68b1
Signed-off-by: Mayank Rana <mrana@codeaurora.org>