r8169: do not abort when the power management capabilities are disabled

The capabilities of the 8169 can be disabled but it is hardly a reason
to prevent the use the device. The (so far) unusual behavior has been
reported on a MIPS platform by Yoichi Yuasa.

Spotted-by: Yoichi Yuasa <yuasa@hh.iij4u.or.jp>
Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>