iio:adc:ti-adc084s021 Tidy up endian types

By adding a few local variables and avoiding a void * for
a parameter we can easily make all the endian types explicit and
get rid of the warnings from sparse:

  CHECK   drivers/iio/adc/ti-adc084s021.c
drivers/iio/adc/ti-adc084s021.c:84:26: warning: incorrect type in assignment (different base types)
drivers/iio/adc/ti-adc084s021.c:84:26:    expected unsigned short [usertype]
drivers/iio/adc/ti-adc084s021.c:84:26:    got restricted __be16
drivers/iio/adc/ti-adc084s021.c:115:24: warning: cast to restricted __be16
drivers/iio/adc/ti-adc084s021.c:115:24: warning: cast to restricted __be16
drivers/iio/adc/ti-adc084s021.c:115:24: warning: cast to restricted __be16
drivers/iio/adc/ti-adc084s021.c:115:24: warning: cast to restricted __be16

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20200722155103.979802-23-jic23@kernel.org