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>