staging: wfx: avoid namespace contamination

staging: wfx: avoid namespace contamination

tx_policy_init() was already defined in driver cw1200. So, compilation
failed when wfx and cw1200 were both built-in.

In order to keep a coherent naming scheme, this patch prefixes all
"tx_policy_*" functions with "wfx_".

Fixes: 9bca45f3d692 ("staging: wfx: allow to send 802.11 frames")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20191008094232.10014-8-Jerome.Pouiller@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>