mlxsw: core_hwmon: Split temperature querying from show functions

mlxsw_hwmon_module_temp_show(), mlxsw_hwmon_module_temp_critical_show()
and mlxsw_hwmon_module_temp_emergency_show() query the relevant
temperature from firmware and fill the value in provided buffers.

Split the temperature querying functionality to individual get()
functions and call them from the show() functions.

The get() functions will be used by subsequent patches in the set.

Signed-off-by: Amit Cohen <amitc@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>