iio: adc: fix warning in Qualcomm PM8xxx HK/XOADC driver

iio: adc: fix warning in Qualcomm PM8xxx HK/XOADC driver

The pm8xxx_get_channel() implementation is unclear, and causes gcc to
suddenly generate odd warnings.  The trigger for the warning (at least
for me) was the entirely unrelated commit 79a4e91d1bb2 ("device.h: Add
__cold to dev_<level> logging functions"), which apparently changes gcc
code generation in the caller function enough to cause this:

  drivers/iio/adc/qcom-pm8xxx-xoadc.c: In function ‘pm8xxx_xoadc_probe’:
  drivers/iio/adc/qcom-pm8xxx-xoadc.c:633:8: warning: ‘ch’ may be used uninitialized in this function [-Wmaybe-uninitialized]
    ret = pm8xxx_read_channel_rsv(adc, ch, AMUX_RSV4,
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
             &read_nomux_rsv4, true);
             ~~~~~~~~~~~~~~~~~~~~~~~
  drivers/iio/adc/qcom-pm8xxx-xoadc.c:426:27: note: ‘ch’ was declared here
    struct pm8xxx_chan_info *ch;
                             ^~

because gcc for some reason then isn't able to see that the termination
condition for the "for( )" loop in that function is also the condition
for returning NULL.

So it's not _actually_ uninitialized, but the function is admittedly
just unnecessarily oddly written.

Simplify and clarify the function, making gcc also see that it always
returns a valid initialized value.

Cc: Joe Perches <joe@perches.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Andy Gross <andy.gross@linaro.org>
Cc: David Brown <david.brown@linaro.org>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: Hartmut Knaack <knaack.h@gmx.de>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>