iio: adc: at91: make the function handle_adc_eoc_trigger() static

The handle_adc_eoc_trigger() in only used in at91_adc.c. So make it
static.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>