IB/srp: Wrap OUI checking for workarounds in helper functions

Wrap the checking for Mellanox and Topspin OUIs to decide whether to
use a workaround into helper functions.  This will make it cleaner to
add a new OUI to check (as we need to do now that some targets with a
Cisco OUI still need the Topspin workarounds).

Signed-off-by: Roland Dreier <rolandd@cisco.com>