powerpc: Make eh value more explicit when using lwarx

Just like the first patch of this series, define a local 'eh' in order
to make the code clearer.

And IS_ENABLED() returns either 1 or 0 so no need to do
IS_ENABLED(CONFIG_PPC64) ? 1 : 0.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[mpe: Use symbolic names, use 'n' constraint per Segher]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/629befaa2d05e2922346e58a383886510d6af55a.1659430931.git.christophe.leroy@csgroup.eu