iio: hrtimer-trigger: Fix potential integer overflow in iio_hrtimer_store_sampling_frequency

Add suffix ULL to constant 1000 in order to avoid a potential integer
overflow and give the compiler complete information about the proper
arithmetic to use. Notice that this constant is being used in a context
that expects an expression of type unsigned long long, but it's
currently evaluated using 32-bit arithmetic.

Addresses-Coverity-ID: 1503062 ("Unintentional integer overflow")
Fixes: dafcf4ed ("iio: hrtimer: Allow sub Hz granularity")
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20210329205817.GA188755@embeddedor


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>