y2038: fix typo in powerpc vdso "LOPART"

The earlier patch introduced a typo, change LOWPART back to
LOPART.

Fixes: 176ed98c ("y2038: vdso: powerpc: avoid timespec references")
Reported-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>