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.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>