bnxt_en: Refactor bnxt_ring_struct.

Move the DMA page table and vmem fields in bnxt_ring_struct to a new
bnxt_ring_mem_info struct.  This will allow context memory management
for a new device to re-use some of the existing infrastructure.

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