iio: buffer: Fix uninitialized variable ret

iio: buffer: Fix uninitialized variable ret

When !iio_buffer_space_available(rb) is true and signal_pending(current)
is false the end of the do-while loop is reached and the uninitialized
variable ret is zero checked. Fix this by initializing variable ret to
zero.

Addresses-Coverity: ("Uninitialized scalar variable")
Fixes: 9eeee3b0bf19 ("iio: Add output buffer support")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20211015153254.33783-1-colin.king@canonical.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>