powerpc/powernv: Simplify pnv_eeh_reset()

This drops unnecessary nested if statements in pnv_eeh_reset() to
improve the code readability. After the changes, the unused local
variable "ret" is dropped as well. No logical changes introduced.

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