xhci: fix port BESL LPM capability checking

Wrong capability bit was checked for best effort service latency.
bit 20 indicate port is BESL LPM capable (BLC),
bit 19 is hardware LPM capable (HLC)

This patch should be backported to kernels as old as 3.11, that
contain the commit a558ccdc

 "usb: xhci:
add USB2 Link power management BESL support"

Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Reported-by: Steve Cotton <steve@s.cotton.clara.co.uk>
Cc: stable@vger.kernel.org