staging: iio: ad9834: use sysfs_emit() and simplify show functions

staging: iio: ad9834: use sysfs_emit() and simplify show functions

Replace sprintf() with sysfs_emit() in sysfs attribute show functions.
sysfs_emit() is the preferred API for sysfs callbacks as it is aware
of the PAGE_SIZE buffer limit.

Also simplify the wavetype_available show functions by removing
the intermediate string variable and returning directly from each
branch.

Signed-off-by: Gabriel Rondon <grondon@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>