net/bnxt: improve small ring sizes support

net/bnxt: improve small ring sizes support

Improve support for small ring sizes:
   - Ensure that transmit free threshold is no more than 1/4 ring size.
   - Ensure that receive free threshold is no more than 1/4 ring size.
   - Validate requested ring sizes against minimum supported size.
   - Use rxq receive free threshold instead of fixed maximum burst
     size to trigger bulk receive buffer allocation.

Reviewed-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Signed-off-by: Lance Richardson <lance.richardson@broadcom.com>