IB/ipath: Add ability to set the LMC via the sysfs debugging interface

This patch adds the ability to set the LMC via a sysfs file as if the SM
sent a SubnSet(PortInfo) MAD.  It is useful for debugging when no SM is
running.

Signed-off-by: Ralph Campbell <ralph.campbell@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>