IB/ehca: Change misleading error message on memory hotplug

The error message printed when the eHCA driver prevents memory hotplug
is misleading -- the user might think that hot-removing the lhca,
hotplugging memory, then hot-adding the lhca again will work, but it
actually doesn't.

Signed-off-by: Joachim Fenkes <fenkes@de.ibm.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>