nitro_enclaves: Fixup type and simplify logic of the poll mask setup

nitro_enclaves: Fixup type and simplify logic of the poll mask setup

Update the assigned value of the poll result to be EPOLLHUP instead of
POLLHUP to match the __poll_t type.

While at it, simplify the logic of setting the mask result of the poll
function.

Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Alexander Graf <graf@amazon.com>
Signed-off-by: Andra Paraschiv <andraprs@amazon.com>
Link: https://lore.kernel.org/r/20201102173622.32169-1-andraprs@amazon.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>