staging: rtl8188eu: remove unused P2P_PRIVATE_IOCTL_SET_LEN

Defined P2P_PRIVATE_IOCTL_SET_LEN is not used in the driver code,
so remove it from wifi.h.

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>