be2net: bug fix in be_change_mtu

Current code allows the new mtu to cross the supported value.
This patch fixes the boundary checks.

From: Suresh R <sureshr@serverengines.com>
Signed-off-by: Ajit Khaparde <ajitk@serverengines.com>
Signed-off-by: David S. Miller <davem@davemloft.net>