net: ncsi: Set ncsi_stop_dev() to inline while NET_NCSI not enabled

While NET_NCSI not enabled, ncsi_stop_dev() is not inline
and call with it, casue compile waring:

linux/include/net/ncsi.h:63:13: warning: 'ncsi_stop_dev'
defined but not used [-Wunused-function]
   static void ncsi_stop_dev(struct ncsi_dev *nd)

Setting ncsi_stop_dev() to inline like other function to
remove compile warnings.

Signed-off-by: Minda Chen <minda.chen@starfivetech.com>
Link: https://patch.msgid.link/20260605033607.37630-1-minda.chen@starfivetech.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/ncsi.h

@@ -60,7 +60,7 @@ static inline int ncsi_start_dev(struct ncsi_dev *nd)
 	return -ENOTTY;
 }
 
-static void ncsi_stop_dev(struct ncsi_dev *nd)
+static inline void ncsi_stop_dev(struct ncsi_dev *nd)
 {
 }