staging: r8188eu: remove empty function

staging: r8188eu: remove empty function

With the RT_TRACE print removed, the rtw_wx_set_freq function is now
empty.

This function should be removed. At the moment, the setfreq ioctl
returns success to the caller although the frequency wasn't set.
It makes more sense to inform callers that this driver does not
support setfreq.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20210807153636.11712-5-martin@kaiser.cx
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>