bnx2x: Adding restriction on sge_buf_size

sge_buff_size may not be more than 0xffff.

Reported-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Vladislav Zolotarov <vladz@broadcom.com>
Tested-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>