drivers/net/ftmac100.c: Fix compile warning

Fix this:
ftmac100.c: In function 'ftmac100_initialize':
ftmac100.c:266:12: warning: assignment from incompatible pointer type

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Cc: Joe Hershberger <joe.hershberger@gmail.com>