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.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/virt/nitro_enclaves/ne_misc_dev.c

@@ -1505,10 +1505,8 @@ static __poll_t ne_enclave_poll(struct file *file, poll_table *wait)
 
 	poll_wait(file, &ne_enclave->eventq, wait);
 
-	if (!ne_enclave->has_event)
-		return mask;
-
-	mask = POLLHUP;
+	if (ne_enclave->has_event)
+		mask |= EPOLLHUP;
 
 	return mask;
 }