ms5611: change bus speed to 5MHz

this gives 5MHz SPI bus speed (by asking for 6MHz due to timer
granularity).

Tests with a logic analyser show that the ms5611 is actually more
reliable at 5MHz than lower speeds

src/drivers/ms5611/ms5611_spi.cpp

@@ -121,7 +121,7 @@ MS5611_spi_interface(ms5611::prom_u &prom_buf)
 }
 
 MS5611_SPI::MS5611_SPI(int bus, spi_dev_e device, ms5611::prom_u &prom_buf) :
-	SPI("MS5611_SPI", nullptr, bus, device, SPIDEV_MODE3, 2000000),
+	SPI("MS5611_SPI", nullptr, bus, device, SPIDEV_MODE3, 6*1000*1000),
 	_prom(prom_buf)
 {
 }