wintun: upgrade error message in case of ring registration failure

wintun: upgrade error message in case of ring registration failure

Rather than have the Interactive Service return a custom 0x20000004
(ERROR_REGISTER_RING_BUFFERS) error, return the true GetLastError() code
that the TUN_IOCTL_REGISTER_RINGS provides.

Signed-off-by: Simon Rozman <simon@rozman.si>
Acked-by: Lev Stipakov <lstipakov@gmail.com>
Message-Id: <20200205183841.1118-1-simon@rozman.si>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg19367.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>