sh: Correct __xdiv64_32/div64_32 return value size.

These should be returning a uint32_t, whereas they were erroneously
returning a u64 before. As the register sizes are 32-bits, this doesn't
really make a lot of sense.

Reported-by: Katsuya MATSUBARA <matsu@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>