powerpc/reg: Remove #ifdef around mtspr macro

That ifdef was introduced by commit 1458dd95 ("powerpc/8xx:
Handle CPU6 ERRATA directly in mtspr() macro") and left over by
commit 2a45addd ("powerpc/8xx: Remove CPU6 ERRATA Workaround")

Remove it.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/cf652e47ea9e453e89813611b6f76d0939a12063.1687344017.git.christophe.leroy@csgroup.eu