bnxt_en: Fix vnic accounting in the bnxt_check_rings() path.

The number of vnics to check must be determined ahead of time because
only standard RX rings require vnics to support RFS.  The logic is
similar to the ring reservation logic and we can now use the
refactored common functions to do most of the work in setting up
the firmware message.

Fixes: 8f23d638 ("bnxt_en: Expand bnxt_check_rings() to check all resources.")
Signed-off-by: Eddie Wai <eddie.wai@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>