xhci: remove the unused sw_lpm_support

It is introduced for the pre-0.96 xHC controllers, and the driver only
support HW LPM for 1.0 and later controllers.It's not actually used now
and is thought not to be used in the future any more, so just remove it.

Signed-off-by: Zeng Tao <prime.zeng@hisilicon.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>