powerpc/boot: Fix a typo in partial_decompress() comment

outbuf is the output buffer, output_size is the size of the output
buffer.

Signed-off-by: Zhang Jianhua <chris.zjh@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210510075134.252978-1-chris.zjh@huawei.com