staging: iio: cdc/ad7746: fix missing return value

As found by "gcc -Wmaybe-uninitialized", the latest change to the
driver lacked an initalization for the return code in one of the
added cases:

drivers/staging/iio/cdc/ad7746.c: In function ‘ad7746_read_raw’:
drivers/staging/iio/cdc/ad7746.c:655:2: error: ‘ret’ may be used uninitialized in this function [-Werror=maybe-uninitialized]

This sets it to IIO_VAL_INT, which I think is what we want here.

Fixes: 2296c062 ("staging: iio: cdc: ad7746: implement IIO_CHAN_INFO_SAMP_FREQ")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>