poll-loop: Ignore 'wevent' in poll_fd_wait_at() on non-Windows.

poll-loop: Ignore 'wevent' in poll_fd_wait_at() on non-Windows.

'wevent' isn't actually used on non-Windows systems, but poll_fd_wait_at()
and find_poll_node() treat events with different 'wevent' as different, so
it seems better to make sure that 'wevent' doesn't matter.

Alternatively, one could ovs_assert(!wevent).  I guess that would catch
a caller accidentally swapping the 'fd' and 'wevent' arguments.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Gurucharan Shetty <gshetty@nicira.com>