bnxt_en: Fix ethtool -l pre-set max combined channel.

With commit d1e7925e ("bnxt_en: Centralize logic to reserve rings."),
ring allocation for combined rings has become stricter.  A combined
ring must now have an rx-tx ring pair.  The pre-set max. for combined
rings should now be min(rx, tx).

Fixes: d1e7925e

 ("bnxt_en: Centralize logic to reserve rings.")
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>