dwc3-msm: Update maximum speed using extcon for high speed case only

EXTCON_PROP_USB_SS property only supports boolean value (i.e 0 - high
speed and 1 - super speed). Hence this property can't be used to
select super speed plus as speed. Hence use hardware supported maximum
speed always and use EXTCON_PROP_USB_SS property based value if it
suggests as high speed.

Change-Id: I157e63bfbb4cf8f756882a7e9573d8c99ea3a73d
Signed-off-by: Mayank Rana <mrana@codeaurora.org>