Update read_reg() to use repeated start

[caternuson]

Possible fix for #37.

Basically just switches to using a repeated start in the read_reg() function.

All of the I2C examples in the MLX90393 datasheet show repeated start. The MLX90393 seems to be somewhat OK with a stop/start split write/read transaction, since that is used in a few places. But that mixed with the additional I2C overhead dealing with setting the muxer does not seem to work (see traces in issue thread).

Tested with QT PY RP2040 + PCA9546A + MLX90393, the same setup that was failing before, works now:

Adafruit CircuitPython 8.2.6 on 2023-09-12; Adafruit QT Py RP2040 with rp2040
>>> import board
>>> import adafruit_mlx90393
>>> import adafruit_tca9548a
>>> i2c = board.STEMMA_I2C()
>>> mux = adafruit_tca9548a.PCA9546A(i2c)
>>> SENSOR = adafruit_mlx90393.MLX90393(mux[2], address=0x0c)
>>> SENSOR.magnetic
(-34.35, -9.9, 78.65)
>>> SENSOR.magnetic
(-34.2, -10.05, 78.65)
>>> SENSOR.magnetic
(-595.95, -68.1, -163.592)
>>>

And still works without the muxer and simply connected directly:

Adafruit CircuitPython 8.2.6 on 2023-09-12; Adafruit QT Py RP2040 with rp2040
>>> import board
>>> import adafruit_mlx90393
>>> i2c = board.STEMMA_I2C()
>>> SENSOR = adafruit_mlx90393.MLX90393(i2c, address=0x0c)
>>> SENSOR.magnetic
(-44.4, 2.25, 55.176)
>>> 

[tannewt]

Makes sense to me! Thank you!