staging: r8188eu: convert rtw_p2p_set_persistent to return void

rtw_p2p_set_persistent always returns 0 and it's return value is not used.
Convert it to return void.

Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Abdun Nihaal <abdun.nihaal@gmail.com>
Link: https://lore.kernel.org/r/2f23981bcdf4177327b9a29e88540380ef627aab.1643466748.git.abdun.nihaal@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>