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>