mlxsw: fix mlxsw_sp2_nve_vxlan_learning_set() return type

The mlxsw_sp2_nve_vxlan_learning_set() function is supposed to return
zero on success or negative error codes.  So it needs to be type int
instead of bool.

Fixes: 4ee70efa ("mlxsw: spectrum_nve: Add support for VXLAN on Spectrum-2")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>