net: switchdev: Simplify 'mlxsw_sp_mc_write_mdb_entry()'

Use 'bitmap_alloc()/bitmap_free()' instead of hand-writing it.
This makes the code less verbose.

Also, use 'bitmap_alloc()' instead of 'bitmap_zalloc()' because the bitmap
is fully overridden by a 'bitmap_copy()' call just after its allocation.

While at it, remove an extra and unneeded space.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>