stm32mp1: remove the board_check_usb_power function when ADC is not activated

Simplify the code of the function board_check_usb_power
based in CONFIG_ADC and adc_measurement; the function is removed by the
linker when the CONFIG_ADC is not activated.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>