drivers/net/wireless/ti/wl1251: remove CONFIG_EXPERIMENTAL

This config item has not carried much meaning for a while now and is
almost always enabled by default. As agreed during the Linux kernel
summit, remove it.

CC: Luciano Coelho <coelho@ti.com>
CC: "John W. Linville" <linville@tuxdriver.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Luciano Coelho <luca@coelho.fi>