console: Fix console_unregister() signature

console_unregister() has always returned a pointer to the console that
was removed on success, not just an integer. Fix the C prototype to
match the assembly implementation.

Change-Id: Iafc43de0767a5c87c9ae5c3aba53761dd28d51e6
Signed-off-by: Julius Werner <jwerner@chromium.org>