[POWERPC] therm_adt746x: Eliminate some build warnings

We don't care if the device_create_file calls fail, the driver will work
just as well without them, so just issue a runtime warning.

drivers/macintosh/therm_adt746x.c: In function 'thermostat_init':
drivers/macintosh/therm_adt746x.c:615: warning: ignoring return value of 'device_create_file', declared with attribute warn_unused_result
drivers/macintosh/therm_adt746x.c:616: warning: ignoring return value of 'device_create_file', declared with attribute warn_unused_result
drivers/macintosh/therm_adt746x.c:617: warning: ignoring return value of 'device_create_file', declared with attribute warn_unused_result
drivers/macintosh/therm_adt746x.c:618: warning: ignoring return value of 'device_create_file', declared with attribute warn_unused_result
drivers/macintosh/therm_adt746x.c:619: warning: ignoring return value of 'device_create_file', declared with attribute warn_unused_result
drivers/macintosh/therm_adt746x.c:620: warning: ignoring return value of 'device_create_file', declared with attribute warn_unused_result
drivers/macintosh/therm_adt746x.c:621: warning: ignoring return value of 'device_create_file', declared with attribute warn_unused_result
drivers/macintosh/therm_adt746x.c:622: warning: ignoring return value of 'device_create_file', declared with attribute warn_unused_result
drivers/macintosh/therm_adt746x.c:623: warning: ignoring return value of 'device_create_file', declared with attribute warn_unused_result
drivers/macintosh/therm_adt746x.c:625: warning: ignoring return value of 'device_create_file', declared with attribute warn_unused_result

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>