powerpc/pseries/vas: Remove the unneeded result variable

Return the value vas_register_coproc_api() 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: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220825072657.229168-1-ye.xingchen@zte.com.cn