USB: xhci: Fix finding extended capabilities registers

According "5.3.6 Capability Parameters (HCCPARAMS)" of xHCI rev0.96 spec,
value of xECP register indicates a relative offset, in 32-bit words,
from Base to the beginning of the first extended capability.
The wrong calculation will cause BIOS handoff fail (not handoff from BIOS)
in some platform with BIOS USB legacy sup support.

Signed-off-by: Edward Shao <laface.tw@gmail.com>
Cc: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>