powerpc/uaccess: Swap clear_user() and __clear_user()

It is clear_user() which is expected to call __clear_user(),
not the reverse.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/d8ec01fb22f33d87321451d5e5f01cb56dacaa39.1615398498.git.christophe.leroy@csgroup.eu