staging: rtl8723bs: remove undefined function prototype in of os_dep/sdio_intf.c

staging: rtl8723bs: remove undefined function prototype in of os_dep/sdio_intf.c

fix the following checkpatch issue:

WARNING: externs should be avoided in .c files
486: FILE: drivers/staging/rtl8723bs/os_dep/sdio_intf.c:486:
+extern int pm_netdev_close(struct net_device *pnetdev, u8 bnormal);

Signed-off-by: Fabio Aiuto <fabioaiuto83@gmail.com>
Link: https://lore.kernel.org/r/06d61465fd33b6cf6d9428ac983e55f50afd448b.1616503354.git.fabioaiuto83@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>