device property: Fix the length used in PROPERTY_ENTRY_STRING()

With string type property entries we need to use
sizeof(const char *) instead of the number of characters as
the length of the entry.

If the string was shorter then sizeof(const char *),
attempts to read it would have failed with -EOVERFLOW. The
problem has been hidden because all build-in string
properties have had a string longer then 8 characters until
now.

Fixes: a85f4204 ("device property: helper macros for property entry creation")
Cc: 4.5+ <stable@vger.kernel.org> # 4.5+
Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>