iio:adc:ti-ads124s08: Fix buffer being too long.

The buffer is expressed as a u32 array, yet the extra space for
the s64 timestamp was expressed as sizeof(s64)/sizeof(u16).
This will result in 2 extra u32 elements.
Fix by dividing by sizeof(u32).

Fixes: e717f8c6

 ("iio: adc: Add the TI ads124s08 ADC code")
Signed-off-by: Jonathan <Cameron&lt;Jonathan.Cameron@huawei.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Cc: Dan Murphy <dmurphy@ti.com>
Cc: <Stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200920112742.170751-8-jic23@kernel.org