3c574, 3c515 bitfields abuse

wn3_config is shared by these cards; the way we deal with it is both bad C
(union abuse) and broken on big-endian.  For 3c515 it's less serious (ISA
cards are quite rare outside of little-endian boxen), but 3c574 is a pcmcia
one and that'd better be endian-independent...  Fix is the same in both
cases.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jeff Garzik <jeff@garzik.org>