Staging: iio: fix endian conversion in ad7298_scan_direct()

"tmp" is used to store the output from cpu_to_be16() so it should be
a __be16 bit type.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>