bnxt_en: Explicitly specify P5 completion rings to reserve

The current code assumes that every RX ring group and every TX ring
requires a completion ring on P5_PLUS chips.  Now that we have the
bnxt_hw_rings structure, add the cp_p5 field so that it can
be explicitly specified.  This makes the logic more clear.

Reviewed-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>