Add support for devices with only one register without an address (e.g. pca9548a / tca9548a)

[the-yrr]

basically what the title says, my current modification is to set the register address to "None" and change the _i2c_read / _i2c_write methods:

def _i2c_write(self, register, value, bit_width):
    values = _int_to_bytes(value, bit_width // self._bit_width, 'big')
    values = list(values)
    if register is None:
        self._i2c.write_byte(self._i2c_address, values[0])
    else:
        self._i2c.write_i2c_block_data(self._i2c_address, register, values)

def _i2c_read(self, register, bit_width):
    value = 0
    if register is None:
        return self._i2c.read_byte(self._i2c_address)
    else:
        for x in self._i2c.read_i2c_block_data(self._i2c_address, register, bit_width // self._bit_width):
            value <<= 8
            value |= x
        return value