tests: drivers: adc_api: use proper printf format

This commit changes the adc_api test to use the proper printf
format specifier when printing the ADC samplings. This ensures
that all values are printed on 16-bit. Without this specifier,
negative values are sign-extended and printed on full integer
size (e.g., 0x8000 -> "0xFFFF8000").

Signed-off-by: Mathieu Choplain <mathieu.choplain@st.com>