powerpc/powernv: Simplify pnv_eeh_set_option()

This simplifies pnv_eeh_set_option() to avoid unnecessary nested
if statements, to improve readability. No functional changes.

Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>