powerpc/32: make the 6xx/8xx EXC_XFER_TEMPLATE() similar to the 40x/booke one

6xx/8xx EXC_XFER_TEMPLATE() macro adds a i##n symbol which is
unused and can be removed.
40x and booke EXC_XFER_TEMPLATE() macros takes msr from the caller
while the 6xx/8xx version uses only MSR_KERNEL as msr value.

This patch modifies the 6xx/8xx version to make it similar to the
40x and booke versions.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>