bnxt_en: Refactor bnxt_need_reserve_rings().

Refactor bnxt_need_reserve_rings() slightly so that __bnxt_reserve_rings()
can call it and remove some duplicated code.

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