atm: zatm: fix memcmp casting

memcmp() returns int, but eprom_try_esi() cast it to unsigned char. One
can lose significant bits and get 0 from non-0 value returned by the
memcmp().

Signed-off-by: Ivan Bornyakov <brnkv.i1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>