drivers: staging: rtl8712: _adapter is declared twice

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

Reviewed-by: Dan Carpenter <[e-mail]>
Signed-off-by: Wan Jiabing <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/rtl8712/drv_types.h

···
 	RTL8716_NIC
 };
 
-struct _adapter;
-
 struct	qos_priv	{
 	/* bit mask option: u-apsd, s-apsd, ts, block ack... */
 	unsigned int qos_option;