cxl: Add missing return statement after handling AFU errror

We were missing a return statement in the PSL interrupt handler in the
case of an AFU error, which would trigger an "Unhandled CXL PSL IRQ"
warning. We do actually handle these type of errors (by notifying
userspace), so add the missing return IRQ_HANDLED so we don't throw
unecessary warnings.

Signed-off-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>