drivers: adc: adc_emul: Move semicolon to allow for multi-instancing

Move semicolon from end of DT_INST_FOREACH to DEVICE_DT_INST_DEFINE
to make multi-instancing possible. This caused some problems with
CI after adding adc node to native_posix dts.

Signed-off-by: Benjamin Björnsson <benjamin.bjornsson@gmail.com>