Returning !bbu_find_handler() from barebox_update_handler_exists()
would return the opposite result from what the name of that funciton
implies. Drop the "!" to make it behave as expected.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>