qtnfmac: switch to kernel circ_buf implementation

Current code for both Rx and Tx queue management is a custom and incomplete
circular buffer implementation. It makes a lot of sense to switch to kernel
built-in circ_buf implementation.

Signed-off-by: Sergey Matyukevich <sergey.matyukevich.os@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>