um: use str_yes_no() to remove hardcoded "yes" and "no"

Remove hard-coded strings by using the str_yes_no() helper function
provided by <linux/string_choices.h>.

Signed-off-by: Ethan Carter Edwards <ethan@ethancedwards.com>
Link: https://patch.msgid.link/20250220-um_yes_no-v1-1-2a355ed2d225@ethancedwards.com


Signed-off-by: Johannes Berg <johannes.berg@intel.com>