iio: dac: ad3552r: fix signedness bug in ad3552r_reset()

The "val" variable is used to store either negative error codes from
ad3552r_read_reg_wrapper() or positive u16 values on success.  It needs
to be signed for the error handling to work correctly.

Fixes: 8f2b5482 ("drivers:iio:dac: Add AD3552R driver support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20220316122354.GA16825@kili


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