bnxt_en: Fix max_mtu setting for multi-buf XDP

The existing code does not allow the MTU to be set to the maximum even
after an XDP program supporting multiple buffers is attached.  Fix it
to set the netdev->max_mtu to the maximum value if the attached XDP
program supports mutiple buffers, regardless of the current MTU value.

Also use a local variable dev instead of repeatedly using bp->dev.

Fixes: 1dc4c557

 ("bnxt: adding bnxt_xdp_build_skb to build skb from multibuffer xdp_buff")
Reviewed-by: Somnath Kotur <somnath.kotur@broadcom.com>
Reviewed-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Reviewed-by: Andy Gospodarek <andrew.gospodarek@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Link: https://lore.kernel.org/r/20230731142043.58855-3-michael.chan@broadcom.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>