spi: ppc4xx: add parenthesis for sizeof

Fix missing parenthesis of sizeof reported by checkpatch.pl:
 WARNING: sizeof *pp should be sizeof(*pp).

The kernel coding style suggests thinking of sizeof as a function
and add parenthesis.

Signed-off-by: Zhiqi Song <songzhiqi1@huawei.com>
Link: https://lore.kernel.org/r/1621301902-64158-8-git-send-email-songzhiqi1@huawei.com
Signed-off-by: Mark Brown <broonie@kernel.org>