net/enic: fix memory freeing

net/enic: fix memory freeing

enic_alloc_consistent() allocated memory, but enic_free_consistent()
was an empty function, so allocated memory was never freed.

This commit adds a list and lock to the enic structure to keep track
of the memzones allocated in enic_alloc_consistent(), and
enic_free_consistent() uses that information to properly free memory.

Fixes: fefed3d1e62c ("enic: new driver")
Signed-off-by: Nelson Escobar <neescoba@cisco.com>
Reviewed-by: John Daley <johndale@cisco.com>