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

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.

Fixes: da35a7b526d9 ("iio: frequency: admv1013: add support for ADMV1013")
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>