staging: rtl8188eu: remove dummy setfreq handler

The setfreq handler contains only a debug print. It returns success
although the frequency wasn't set.

Remove this handler and inform callers that this driver does not
support setfreq.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20210529121346.8422-3-martin@kaiser.cx


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