bnxt_en: Enhance bnxt_alloc_ring()/bnxt_free_ring().

To support level 2 context page memory structures, enhance the
bnxt_ring_mem_info structure with a "depth" field to specify the page
level and add a flag to specify using full pages for L1 and L2 page
tables.  This is needed to support RDMA functionality on 57500 chips
since RDMA requires more context memory.

Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>