drivers/hwmon/adcxx.c: fix for single-channel ADCs

While testing an ADC121S021 in an embedded board with a S3C2142 SoC (ARM
core), I have found that the 'adcxx' driver does not handle correctly
single channel ADCs from this chip family.  For single channel chips you
must only issue one read transfer for correct measurement.

Signed-off-by: Jose Miguel Goncalves <jose.goncalves@inov.pt>
Cc: Marc Pignat <marc.pignat@hevs.ch>
Cc: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>