IPoIB/cm: Fix warning if IPV6 is not enabled

Fix

    drivers/infiniband/ulp/ipoib/ipoib_cm.c:1151: warning: unused variable 'dev'

by getting rid of the variable dev, which is only used if CONFIG_IPV6
is enabled, and replacing the one use of it with the value it is
assigned, namely priv->dev.

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