spi_temperature: Provide a default value for spi_speed

All of the Maxim chips support a 4Mhz spi transfer rate, so use that
as a default spi speed.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

test/klippy/temperature.cfg

@@ -59,7 +59,6 @@ min_temp: 0
 max_temp: 100
 control: watermark
 sensor_type: MAX6675
-spi_speed: 100000
 sensor_pin: ar2
 
 [temperature_fan test_max31855]
@@ -68,7 +67,6 @@ min_temp: 0
 max_temp: 100
 control: watermark
 sensor_type: MAX31855
-spi_speed: 100000
 sensor_pin: ar5
 
 [temperature_fan test_max31856]
@@ -77,7 +75,6 @@ min_temp: 0
 max_temp: 100
 control: watermark
 sensor_type: MAX31856
-spi_speed: 100000
 sensor_pin: ar7
 
 [temperature_fan test_max31865]
@@ -86,7 +83,6 @@ min_temp: 0
 max_temp: 100
 control: watermark
 sensor_type: MAX31865
-spi_speed: 100000
 sensor_pin: ar13
 
 [thermistor my_custom_thermistor]