iio: adc: ina2xx: Remove unneeded dummy read to clear CNVR flag

iio: adc: ina2xx: Remove unneeded dummy read to clear CNVR flag

Although the datasheet states the CNVR flag is cleared by reading the
BUS_VOLTAGE register, it is actually cleared by reading any of the
voltage/current/power registers.

The behaviour has been confirmed by TI support:
http://e2e.ti.com/support/amplifiers/current-shunt-monitors/f/931/p/647053/2378282

Signed-off-by: Stefan Brüns <[email protected]>
Signed-off-by: Jonathan Cameron <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>