net/bnxt: fix MTU calculation

net/bnxt: fix MTU calculation

We were not considering the case of nested VLANs while
calculating MTU. This patch takes care of the same.

Fixes: daef48efe5e5 ("net/bnxt: support set MTU")
Cc: stable@dpdk.org
Signed-off-by: Qingmin Liu <qingmin.liu@broadcom.com>
Signed-off-by: Scott Branden <scott.branden@broadcom.com>
Reviewed-by: Jay Ding <jay.ding@broadcom.com>
Reviewed-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Reviewed-by: Randy Schacher <stuart.schacher@broadcom.com>
Signed-off-by: Ajit Khaparde <ajit.khaparde@broadcom.com>