gianfar: Do right check on num_txbdfree

This patch fixes a wrong check on num_txbdfree. It could lead to
num_txbdfree become nagative.  Result was that the gianfar stops
sending data.

Changes from first version :
- removed a space between parens (David Millers comment)
- full email address in signed off line

Signed-off-by: Rini van Zetten <rini@arvoo.nl>
Acked-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>