cs5520: add enablebits checking

Based on sparse comments in OpenFirmware code
(no Cx5510/Cx5520 datasheet here).

This fixes 2.6.26 regression reported by TAKADA
and caused by addition of warm-plug support.

Tested-by: TAKADA Yoshihito <takada@mbf.nifty.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>