powerpc: Use rol32() instead of opencoding in csum_fold()

rol32(x, 16) will do the rotate using rlwinm.

No need to open code using inline assembly.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/794337eff7bb803d2c4e67d9eee635390c4c48fe.1646812553.git.christophe.leroy@csgroup.eu