[gardena] Improve GardenaSmartImpl initialization order (#14722)

[gardena] Improve GardenaSmartImpl initialization order (#14722)

If an exception occurs during httpClient.start(), webSocketClient
remained null, which led to another (then unhandled) exception being thrown
by dispose(), called in the exception handler.
With the new initialization order it's ensured webSocketClient is not
null at dispose() time, thus allowing proper propagation of the
exception thrown at HTTP startup.

Signed-off-by: Danny Baumann <dannybaumann@web.de>