qla3xxx: Remove API to change MTU.

This network device driver shares the same hardware as the qla4xxx
iSCSI driver. Changing the MTU via the device interface will
cause qla4xxx to crash as there is no way to make notification.
Users wishing to change the MTU must do so using an iSCSI
utility such as Qlogic SanSurfer. This forces the user to
unload/reload this network device driver after the MTU
value has been changed in flash.

Signed-off-by: Ron Mercer <ron.mercer@qlogic.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>