MIPS: TXx9: Disable PM capability of TX493[89] internal ether

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: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>