powerpc/powernv: Fix minor off-by-one error in opal_mce_check_early_recovery()

Fix off-by-one error in opal_mce_check_early_recovery() when checking
whether the NIP falls within OPAL space.

Signed-off-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>