net: hsr: fix NULL checks in the code

This patch replaces all instance of NULL checks such as
    if (foo == NULL) with if (!foo)
Also
    if (foo != NULL) with if (foo)

This is seen when ran checkpatch.pl -f on files under net/hsr
and suggestion is to replace as above.

Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>