net/bnxt: disallow MTU change when device is started

net/bnxt: disallow MTU change when device is started

With this patch, bnxt_mtu_set_op() will return an error code if the
device has already started. The user application will have to take
care to bring down device before invoking the mtu_set()

Fixes: daef48efe5e5 ("net/bnxt: support set MTU")
Cc: stable@dpdk.org
Signed-off-by: Damodharam Ammepalli <damodharam.ammepalli@broadcom.com>
Reviewed-by: Andy Gospodarek <andrew.gospodarek@broadcom.com>
Reviewed-by: Ajit Khaparde <ajit.khaparde@broadcom.com>