bnxt_en: Centralize logic to reserve rings.

Currently, bnxt_setup_tc() and bnxt_set_channels() have similar and
duplicated code to check and reserve rx and tx rings.  Add a new
function bnxt_reserve_rings() to centralize the logic.  This will
make it easier to add XDP_TX support which requires allocating a
new set of TX rings.

Also, the tx ring checking logic in bnxt_setup_msix() can be removed.
The rings have been reserved before hand.

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