soc: apple: mailbox: error pointers are negative integers

In an entirely unrelated discussion where I pointed out a stupid thinko
of mine, Rasmus piped up and noted that that obvious mistake already
existed elsewhere in the kernel tree.

An "error pointer" is the negative error value encoded as a pointer,
making the whole "return error or valid pointer" use-case simple and
straightforward.  We use it all over the kernel.

But the key here is that errors are _negative_ error numbers, not the
horrid UNIX user-level model of "-1 and the value of 'errno'".

The Apple mailbox driver used the positive error values, and thus just
returned invalid normal pointers instead of actual errors.

Of course, the reason nobody ever noticed is that the errors presumably
never actually happen, so this is fixing a conceptual bug rather than an
actual one.

Reported-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Link: https://lore.kernel.org/all/5c30afe0-f9fb-45d5-9333-dd914a1ea93a@prevas.dk/


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>