mlxsw: Add a dedicated structure for bitmap of ports

Currently when bitmap of ports is needed, 'unsigned long *' type is
used. The functions which use the bitmap assume its length according to
its name, i.e., each function which gets a bitmap of ports queries the
maximum number of ports and uses it as the size.

As preparation for the next patch which will use bitmap of ports, add a
dedicated structure for it. Refactor the existing code to use the new
structure.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>