Fix bugprone-string-integer-assignment warnings

* mdns.cpp:153:20: warning: an integer is interpreted as a character code
  when assigning it to a string; if this is intended, cast the integer to
  the appropriate character type; if you want a string representation, use
  the appropriate conversion facility [bugprone-string-integer-assignment]

Test: WITH_TIDY=1 make
Change-Id: Id9a790ac31722c6ee8886703939977b913ce95fe