intel_scu_ipc: fix signedness bug

busy_loop() returns negative error code, thus change err variable
from u32 to int to properly propagate correct error code.

Also remove unneeded initialization for err and i variables.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>