atm: br2684: Avoid alignment issues

Use memcmp() instead of cast to u16 when checking the PAD field.

Signed-off-by: Pascal Hambourg <pascal@plouf.fr.eu.org>
Signed-off-by: chas williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Signed-off-by: David S. Miller <davem@davemloft.net>