sparc: sbus: treat CPU index as integer

Using a `char` to fit a CPU index is too small, and assigning -1 to it
isn't correct either, since `char` is sometimes signed and sometimes
not. In this case, it's been fine since this driver only works on sparc,
but that will soon be changing when chars become unsigned everywhere. So
instead, use a normal `int` type, which matches the `int cpu` function
argument that this is being compared against.

Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>