atm: clean up vcc_seq_next()

It's confusing to call PTR_ERR(v).  The PTR_ERR() function is basically
a fancy cast to long so it makes you wonder, was IS_ERR() intended?  But
that doesn't make sense because vcc_walk() doesn't return error
pointers.

This patch doesn't affect runtime, it's just a cleanup.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>