drivers: staging: rtl8712: _adapter is declared twice

struct _adapter has been declared at 23rd line.
Remove the duplicate.

Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Link: https://lore.kernel.org/r/20210325080050.861273-1-wanjiabing@vivo.com


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