mlxsw: thermal: Fix null dereference of NULL temperature parameter

The call to mlxsw_thermal_module_temp_and_thresholds_get passes a NULL
pointer for the temperature and this can be dereferenced in this function
if the mlxsw_reg_query call fails.  The simplist fix is to pass the
address of dummy temperature variable instead of a NULL pointer.

Addresses-Coverity: ("Explicit null dereferenced")
Fixes: 72a64c2f ("mlxsw: thermal: Read module temperature thresholds using MTMP register")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>