Disable port multiplier support on Marvell 88SE61xx chips.

According to report, some recent unrelated changes in the driver triggered
timeouts when testing for absent port multiplier. Cause of this behavior
channge is unclear, but since these chips are old, rare and buggy, it is
easier to just disable port multiplier support, same as done in Linux.

Reported by: bar
MFC after: 3 days