iio: invensense: Prevent possible integer overflow while multiplication

Typecast a variable to int64_t for 64-bit arithmetic multiplication.

Signed-off-by: Karan Sanghavi <karansanghvi98@gmail.com>
Link: https://scan7.scan.coverity.com/#/project-view/51946/11354?selectedIssue=1586045
Link: https://patch.msgid.link/20241103-coverity1586045integeroverflow-v1-1-43ea37a3f3cd@gmail.com


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