IXP4xx: Prevent HSS transmitter lockup by disabling FRaMe signals.

IXP4xx: Prevent HSS transmitter lockup by disabling FRaMe signals.

With port configured with PCR_FRM_SYNC_OUTPUT* and external clock, bringing
the hdlcX interface up and down without active clock supplied to the HSS
causes a TX lockup. We don't support channelized/partial interfaces so
FRaMe signals can't be used anyway, disabling them makes the lockup go away.

Changes to this logic will be required if we want to support channelized
HSS mode (this is most probably bug in NPE-A HSS firmware).

Signed-off-by: Krzysztof Hałasa <khc@pm.waw.pl>