tg3: Raise the jumbo frame BD flag threshold

The current transmit routines set the jumbo frame BD flag too
aggressively.  This can reduce performance for common cases.  This patch
raises the jumbo flag threshold to 1518, up from 1500.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Benjamin Li <benli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>