net: atlantic: Use the bitmap API instead of hand-writing it

Simplify code by using bitmap_weight() and bitmap_zero() instead of
hand-writing these functions.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Igor Russkikh <irusskikh@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>