iio: adc: stm32-adc: fix kernel-doc warnings

Fix the following warnings when documentation is built:
drivers/iio/adc/stm32-adc-core.c:62: warning: cannot understand function
 prototype: 'struct stm32_adc_common_regs '
drivers/iio/adc/stm32-adc-core.c:78: warning: cannot understand function
 prototype: 'struct stm32_adc_priv_cfg '
drivers/iio/adc/stm32-adc-core.c:123: warning: Function parameter or
 member 'pdev' not described in 'stm32f4_adc_clk_sel'
drivers/iio/adc/stm32-adc.c:219: warning: cannot understand function
 prototype: 'struct stm32_adc_regs '
drivers/iio/adc/stm32-adc.c:237: warning: cannot understand function
 prototype: 'struct stm32_adc_regspec '
drivers/iio/adc/stm32-adc.c:264: warning: cannot understand function
 prototype: 'struct stm32_adc_cfg '
drivers/iio/adc/stm32-adc.c:323: warning: Function parameter or member
 'difsel' not described in 'N'
drivers/iio/adc/stm32-adc.c:323: warning: Function parameter or member
 'pcsel' not described in 'stm32_adc'
drivers/iio/adc/stm32-adc.c:371: warning: cannot understand function
 prototype: 'const struct stm32_adc_regs stm32f4_sq[STM32_ADC_MAX_SQ + 1]
drivers/iio/adc/stm32-adc.c:417: warning: cannot understand function
 prototype: 'const struct stm32_adc_regs stm32f4_smp_bits[] = '
drivers/iio/adc/stm32-adc.c:508: warning: cannot understand function
 prototype: 'const struct stm32_adc_regs stm32h7_smp_bits[] = '
drivers/iio/adc/stm32-adc.c:1112: warning: Function parameter or member
 'indio_dev' not described in 'stm32_adc_get_trig_extsel'
drivers/iio/adc/stm32-adc.c:1420: warning: Function parameter or member
 'indio_dev' not described in 'stm32_adc_debugfs_reg_access'
drivers/iio/adc/stm32-adc.c:1420: warning: Function parameter or member
 'reg' not described in 'stm32_adc_debugfs_reg_access'
drivers/iio/adc/stm32-adc.c:1420: warning: Function parameter or member
 'writeval' not described in 'stm32_adc_debugfs_reg_access'
drivers/iio/adc/stm32-adc.c:1420: warning: Function parameter or member
 'readval' not described in 'stm32_adc_debugfs_reg_access'

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>