kprobes/arm: fix decoding of arithmetic immediate instructions

The ARM kprobes arithmetic immediate instruction decoder
(space_cccc_001x()) was accidentally zero'ing out not only the Rn and
Rd arguments, but the lower nibble of the immediate argument as well
-- this patch fixes this.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Acked-by: Nicolas Pitre <nico@marvell.com>