net: caif: use max() to simplify the code

When processing the tail append of sk buffer, the final length needs
to be determined based on expectlen and addlen. Using max() here can
increase the readability of the code.

Signed-off-by: Li Zetao <lizetao1@huawei.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20240822133908.1042240-4-lizetao1@huawei.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>