cnic: tidy up a size calculation

Static checkers complain that id_tbl->table points to longs and 4 bytes
is smaller than sizeof(long).  But the since other side is dividing by
32 instead of sizeof(long), that means the current code works fine.

Anyway, it's more conventional to use the BITS_TO_LONGS() macro when
we're allocating a bitmap.

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