bnxt_en: Improve preset max value for ethtool -l.

The current logic that calculates the preset maximum value for combined
channel does not take into account the rings used for XDP and mqprio
TCs.  Each of these features will reduce the number of TX rings.  Add
the logic to divide the TX rings accordingly based on whether the
device is currently in XDP mode and whether TCs are in use.

Reviewed-by: Vasundhara Volam <vasundhara-v.volam@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>