[ipoib] Mark REMAC cache as expensive

As with the neighbour cache, discarding an REMAC cache entry is
potentially very disruptive.

Originally-fixed-by: Wissam Shoukair <wissams@mellanox.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/net/ipoib.c

 }
 
 /** IPoIB cache discarder */
-struct cache_discarder ipoib_discarder __cache_discarder ( CACHE_NORMAL ) = {
+struct cache_discarder ipoib_discarder __cache_discarder ( CACHE_EXPENSIVE ) = {
 	.discard = ipoib_discard_remac,
 };