rtlwifi: Remove debugging entry in sysfs

As the kernel provides access to module parameters through entries in
/sys/module/<driver>/parameters/, there is no need for a private
interface. Thus the existing code for setting the debug level is
removed.

Reported-by: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>