crypto: octeontx2 - Remove the unneeded result variable

Return the value otx2_cpt_send_mbox_msg() directly instead of storing it
in another redundant variable.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: ye xingchen <ye.xingchen@zte.com.cn>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>