mlxsw: spectrum: Refactor LAG create and destroy code

mlxsw_sp stores an array of LAGs. When a port joins a LAG, in case that
this LAG is already in use, we only have to increase the reference counter.
Otherwise, we have to search for an unused LAG ID and configure it in
hardware. When a port leaves a LAG, we have to destroy it only for the last
user. This code can be simplified, for such requirements we usually add
get() and put() functions which create and destroy the object.

Add mlxsw_sp_lag_{get,put}() and use them. These functions take care of
the reference counter and hardware configuration if needed. Change the
reference counter to refcount_t type which catches overflow and underflow
issues.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>