iio: staging: ad7298_ring: Fix maybe-uninitialized warning

drivers/staging/iio/adc/ad7298_ring.c:97:37: warning: 'time_ns' may
be used uninitialized in this function [-Wmaybe-uninitialized]

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/staging/iio/adc/ad7298_ring.c

@@ -76,7 +76,7 @@ static irqreturn_t ad7298_trigger_handler(int irq, void *p)
 	struct iio_dev *indio_dev = pf->indio_dev;
 	struct ad7298_state *st = iio_priv(indio_dev);
 	struct iio_buffer *ring = indio_dev->buffer;
-	s64 time_ns;
+	s64 time_ns = 0;
 	__u16 buf[16];
 	int b_sent, i;