mlxsw: core: mlxsw: core: avoid -Wint-in-bool-context warning

mlxsw: core: mlxsw: core: avoid -Wint-in-bool-context warning

A recently added function in mlxsw triggers a harmless compiler warning:

In file included from drivers/net/ethernet/mellanox/mlxsw/core.h:17,
                 from drivers/net/ethernet/mellanox/mlxsw/core_env.c:7:
drivers/net/ethernet/mellanox/mlxsw/core_env.c: In function 'mlxsw_env_module_temp_thresholds_get':
drivers/net/ethernet/mellanox/mlxsw/reg.h:8015:45: error: '*' in boolean context, suggest '&&' instead [-Werror=int-in-bool-context]
 #define MLXSW_REG_MTMP_TEMP_TO_MC(val) (val * 125)
                                        ~~~~~^~~~~~
drivers/net/ethernet/mellanox/mlxsw/core_env.c:116:8: note: in expansion of macro 'MLXSW_REG_MTMP_TEMP_TO_MC'
   if (!MLXSW_REG_MTMP_TEMP_TO_MC(module_temp)) {
        ^~~~~~~~~~~~~~~~~~~~~~~~~

The warning is normally disabled, but it would be nice to enable
it to find real bugs, and there are no other known instances at
the moment.

Replace the negation with a zero-comparison, which also matches
the comment above it.

Fixes: d93c19a1d95c ("mlxsw: core: Add API for QSFP module temperature thresholds reading")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>