rndis_wlan: fix sparse warnings

It is obviously wrong to use an enum in a little endian struct,
and those other enums should be declared differently.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>