bnxt_en: Increase RING_IDLE minimum threshold to 50

This keeps the RING_IDLE flag set in hardware for higher coalesce
settings by default and improved latency.

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