ocxl: Remove the unneeded result variable

Return the value opal_npu_spa_clear_cache() 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>
Acked-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220906072006.337099-1-ye.xingchen@zte.com.cn