iio: frequency: admv1013: fix NULL pointer dereference on str

In the Linux kernel, the following vulnerability has been resolved: iio: frequency: admv1013: fix NULL pointer dereference on str When device_property_read_string() fails, str is left uninitialized but the code falls through to strcmp(str, ...), dereferencing a garbage pointer. Replace manual read/strcmp with device_property_match_property_string() and consolidate the SE mode enums into a single sequential enum, mapping to hardware register values via a switch consistent with other bitfields in the driver. Several cleanup patches have been applied to this driver recently so this will need a manual backport.