iio: adc: ti_am335x_adc: Limit step_avg to valid range for gcc complains

FIELD_PREP() checks that a value fits into the available bitfield, add a
check for step_avg to fix gcc complains.

which gcc complains about:
drivers/iio/adc/ti_am335x_adc.c: In function 'tiadc_step_config':
include/linux/compiler_types.h:572:38: error: call to
'__compiletime_assert_491' declared with attribute error: FIELD_PREP: value
too large for the field include/linux/mfd/ti_am335x_tscadc.h:58:29: note:
in expansion of macro 'FIELD_PREP'
#define STEPCONFIG_AVG(val) FIELD_PREP(GENMASK(4, 2), (val))
^~~~~~~~~~
drivers/iio/adc/ti_am335x_adc.c:127:17: note: in expansion of macro 'STEPCONFIG_AVG'
stepconfig = STEPCONFIG_AVG(ffs(adc_dev->step_avg[i]) - 1)

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202510102117.Jqxrw1vF-lkp@intel.com/
Signed-off-by: Pei Xiao <xiaopei01@kylinos.cn>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>