IB/core: Fix oops in ib_cache_gid_set_default_gid

When we fail to find the default gid index, we can't continue
processing in this routine or else we will pass a negative
index to later routines resulting in invalid memory access
attempts and a kernel oops.

Fixes: 03db3a2d (IB/core: Add RoCE GID table management)
Signed-off-by: Doug Ledford <dledford@redhat.com>