staging: rtl8188eu: Replace one-element array with flexible-array in struct ndis_802_11_var_ie

There is a regular need in the kernel to provide a way to declare having
a dynamically sized set of trailing elements in a structure. Kernel code
should always use “flexible array members”[1] for these cases. The older
style of one-element or zero-length arrays should no longer be used[2].

Use flexible-array member in struct ndis_802_11_var_ie, instead of
one-element array.

Also, this helps with the ongoing efforts to enable -Warray-bounds by
fixing the following warning:

drivers/staging/rtl8188eu/core/rtw_wlan_util.c: In function ‘HT_caps_handler’:
drivers/staging/rtl8188eu/core/rtw_wlan_util.c:665:65: warning: array subscript 2 is above array bounds of ‘u8[1]’ {aka ‘unsigned char[1]’} [-Warray-bounds]
  665 |    if ((pmlmeinfo->HT_caps.ampdu_params_info & 0x3) > (pIE->data[i] & 0x3))
      |                                                        ~~~~~~~~~^~~

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://www.kernel.org/doc/html/v5.9/process/deprecated.html#zero-length-and-one-element-arrays

Link: https://github.com/KSPP/linux/issues/79
Link: https://github.com/KSPP/linux/issues/109Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20210304020228.GA45139@embeddedorSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/rtl8188eu/include/wlan_bssdef.h

@@ -64,7 +64,7 @@ struct ndis_802_11_fixed_ie {
 struct ndis_802_11_var_ie {
 	u8  ElementID;
 	u8  Length;
-	u8  data[1];
+	u8  data[];
 };
 
 /*