AtPhonebook handleCpbsCommand was incorrectly sending two responses
to some requests (one inside the swtich statement, then repeating it
after breaking out of the switch statement).
This broke phonebook sync with Audi 2G MMI car kit.
Signed-off-by: Nigel Hannam <nigel@hannam.eu>

Change-Id: I65e54400aad2413725469951899d041870691369