bnxt_en: Refactor statistics code and structures.

The driver manages multiple statistics structures of different sizes.
They are all allocated, freed, and handled practically the same.  Define
a new bnxt_stats_mem structure and common allocation and free functions
for all staistics memory blocks.

Reviewed-by: Vasundhara Volam <vasundhara-v.volam@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>