mlxsw: Fix firmware flashing

The commit cited below moved firmware flashing functionality from
mlxsw_spectrum to mlxsw_core, but did not adjust the Kconfig
dependencies. This makes it possible to have mlxsw_core as built-in and
mlxfw as a module. The mlxfw code is therefore not reachable from
mlxsw_core and firmware flashing fails:

# devlink dev flash pci/0000:01:00.0 file mellanox/mlxsw_spectrum-13.2008.1310.mfa2
devlink answers: Operation not supported

Fix by having mlxsw_core select mlxfw.

Fixes: b79cb787 ("mlxsw: Move fw flashing code into core.c")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reported-by: Vadim Pasternak <vadimp@nvidia.com>
Tested-by: Vadim Pasternak <vadimp@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>