usb: ehci: prevent bad PORTSC register access

usb: ehci: prevent bad PORTSC register access

1. The 'index' of ehci_submit_root() is not always > 0.

   e.g.
   While it gets invoked from usb_get_descriptor(),
   the 'index' is always a '0'. (See ch.9 of USB2.0)

2. The PORTSC register is not always required, and thus it
   should only report a port error when necessary.
   It would cause a port scan failure if the ehci_submit_root()
   always gets terminated by a port error.

Signed-off-by: Kuo-Jung Su <dantesu@faraday-tech.com>
CC: Marek Vasut <marex@denx.de>