IB/core: Create common start/end port functions

IB/core: Create common start/end port functions

commit 0cf18d7723055709faf51b50f5a33253b480637f upstream.

Previously start_port and end_port were defined in 2 places, cache.c and
device.c and this prevented their use in other modules.

Make these common functions, change the name to reflect the rdma
name space, and update existing users.

Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>
[bwh: Backported to 3.2: drop one inapplicable change]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>