rtlwifi: use sk_buff to queue C2H commands

We use 'struct rtl_c2hcmd' to store C2H commands originally, and the code
is slightly complex to enqueue and dequeue and also wastes time to
allocate and memcpy data. Since C2H commands are asynchronous events,
they can be processed in work queue, so RX ISR enqueues C2H result in
removal of rtl_c2h_packet_handler().

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>