GenWQE: Do not modify return code of genwqe_set_interrupt_capability

Follow up patch to the one from Sebastian Ott. There is no need to
change the return code once it fails. And Sebastians version is tested
now and works nicely on our test-system.

Signed-off-by: Frank Haverkamp <haver@linux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>