adc: meson-saradc: skip hardware init only if ADC is enabled

The driver skips hardware initialization if it is already configured by
the earlier bootloader stage (BL30). Skip the initialization only if the
hardware is really initialized and enabled.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>