IB/ipath: Make handling of one subport consistent

Previously the driver and userspace code handled the case of 1 subport
somewhat inconsistently.  The new interpretation of this situation is
that if one subport is requested, the driver turns on the subport
mechanism and arranges for the port to be "shared" by one process.  In
normal use the userspace library does not use this configuration and
instead arranges for the port not to be shared at all.  This
particular idiom can be useful for testing purposes.

Signed-off-by: Mark Debbage <mark.debbage@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>