net: thunderx: avoid null pointer dereference

In function bgx_lmac_handler only use a member of
lmac after checking it is not null.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: David Daney <david.daney@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>