of: correct of_console_check()'s return value

The comments above of_console_check() say that it will return TRUE if it
registers a preferred console, but add_preferred_console() uses a
0-equals-success convention, so this leaves of_console_check() with an
inconsistent policy for its return values.

Fortunately, nobody was actually checking the return value of
of_console_check(), so this isn't significant at the moment.

But let's match the comments, so we're doing what we say.

Fixes: 3482f2c5

 ('of: Create of_console_check() for selecting a console specified in /chosen')
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Grant Likely <grant.likely@linaro.org>