Rename wcn_sta to wcn36xx_sta

Use the same prefix everywhere.

Signed-off-by: Eugene Krasnikov <k.eugene.e@gmail.com>
Move sta_index to wcn36xx_sta struct

sta_index and dpu_desc_index belong to wcn36xx_sta so move it
to this structure. But bss_sta_index must be stored as well.
Depends on the mode sometimes bss_sta_index must be used and
sometimes sta_index.

Signed-off-by: Eugene Krasnikov <k.eugene.e@gmail.com>