iio: adc: stmpe-adc: Cleanup endian type of local variable

Nothing stops data being of type __be16, which fixes the warning:

CHECK   drivers/iio/adc/stmpe-adc.c
drivers/iio/adc/stmpe-adc.c:202:29: warning: cast to restricted __be16
drivers/iio/adc/stmpe-adc.c:202:29: warning: cast to restricted __be16
drivers/iio/adc/stmpe-adc.c:202:29: warning: cast to restricted __be16
drivers/iio/adc/stmpe-adc.c:202:29: warning: cast to restricted __be16

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by:  <philippe.schenker@toradex.com>