powerpc/uaccess: get rid of small constant size cases in raw_copy_{to,from}_user()

Copied from commit 4b842e4e ("x86: get rid of small
constant size cases in raw_copy_{to,from}_user()")

Very few call sites where that would be triggered remain, and none
of those is anywhere near hot enough to bother.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/99d4ccb58a20d8408d0e19874393655ad5b40822.1612879284.git.christophe.leroy@csgroup.eu