brcmfmac: replace strncpy() by strscpy()

The strncpy() may truncate the copied string,
replace it by the safer strscpy().

To avoid below compile warning with gcc 8.2:

drivers/net/wireless/broadcom/brcm80211/brcmfmac/cfg80211.c:In function 'brcmf_vndr_ie':
drivers/net/wireless/broadcom/brcm80211/brcmfmac/cfg80211.c:4227:2:
warning: 'strncpy' output truncated before terminating nul copying 3 bytes from a string of the same length [-Wstringop-truncation]
  strncpy(iebuf, add_del_cmd, VNDR_IE_CMD_LEN - 1);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Signed-off-by: Xulin Sun <xulin.sun@windriver.com>
Acked-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/broadcom/brcm80211/brcmfmac/cfg80211.c

@@ -4244,9 +4244,7 @@ brcmf_parse_vndr_ies(const u8 *vndr_ie_buf, u32 vndr_ie_len,
 static u32
 brcmf_vndr_ie(u8 *iebuf, s32 pktflag, u8 *ie_ptr, u32 ie_len, s8 *add_del_cmd)
 {
-
-	strncpy(iebuf, add_del_cmd, VNDR_IE_CMD_LEN - 1);
-	iebuf[VNDR_IE_CMD_LEN - 1] = '\0';
+	strscpy(iebuf, add_del_cmd, VNDR_IE_CMD_LEN);
 
 	put_unaligned_le32(1, &iebuf[VNDR_IE_COUNT_OFFSET]);