macintosh/macio-adb: Fix warning comparing pointer to 0

Avoid pointer type value compared with 0 to make code clear.

Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1647509089-4280-1-git-send-email-baihaowen@meizu.com