staging: wfx: relocate wfx_fill_sl_key() in secure_link.h

staging: wfx: relocate wfx_fill_sl_key() in secure_link.h

"Secure link" feature is not available in in-tree driver (because it
depends on mbedtls). Thus, secure_link.h only empty functions.

Module parameter "slk_key" and associated function wfx_fill_sl_key() had
an unjustifiable place in main.c. This patch relocate them to
secure_link.h.

BTW, content of wfx_fill_sl_key() is now useless. Just keep a warning if
user try to use "slk_key" attribute (unsupported by this driver).

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20191017093954.657-2-Jerome.Pouiller@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>