bnxt_en: Refactor bnxt_alloc_stats().

Reverse the condition of the large "if" block and return early.  This
will simplify the follow up patch to add PCIe statistics.

Signed-off-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>