Some TC35815 variants (i.e. TX493[89] internal ether) report existance of
PM registers though they are not supported.  Disable PM features by
clearing pdev->pm_cap.
Signed-off-by: NAtsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: NRalf Baechle <ralf@linux-mips.org>