drivers display_nrf_led_matrix: Init variable to avoid compile warning

Initialize row_index to avoid a compile warning.
The warning seems incorrect, as row_index is initialized in the first
interation of the loop, but the overhead of initializing it is trivial,
so we may as well just do it.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>

drivers/display/display_nrf_led_matrix.c

@@ -339,7 +339,7 @@ static void timer_irq_handler(void *arg)
 	const struct display_drv_config *dev_config = dev->config;
 	uint8_t iteration = dev_data->iteration;
 	uint8_t pixel_idx;
-	uint8_t row_idx;
+	uint8_t row_idx = 0;
 
 	/* The timer is automagically stopped and cleared by shortcuts
 	 * on the same event (COMPARE0) that generates this interrupt.