staging: r8188eu: Silence out-of-bounds warning in HT_caps_handler()

Silence the following out-of-bounds warning (caught with -Warray-bounds=2):

drivers/staging/r8188eu/core/rtw_wlan_util.c: In function ‘HT_caps_handler’:
drivers/staging/r8188eu/core/rtw_wlan_util.c:719:54: error: array subscript 2 is above array bounds of ‘u8[1]’ {aka ‘unsigned char[1]’} [-Werror=array-bounds]
  719 |                                             pIE->data[i] & 0x3);
      |                                             ~~~~~~~~~^~~
./include/linux/minmax.h:32:39: note: in definition of macro ‘__cmp_once’
   32 |                 typeof(y) unique_y = (y);               \
      |                                       ^
./include/linux/minmax.h:45:25: note: in expansion of macro ‘__careful_cmp’
   45 | #define min(x, y)       __careful_cmp(x, y, <)
      |                         ^~~~~~~~~~~~~
drivers/staging/r8188eu/core/rtw_wlan_util.c:718:41: note: in expansion of macro ‘min’
  718 |                         max_AMPDU_len = min(pmlmeinfo->HT_caps.u.HT_cap_element.AMPDU_para & 0x3,
      |                                         ^~~
In file included from drivers/staging/r8188eu/core/../include/drv_types.h:16,
                 from drivers/staging/r8188eu/core/rtw_wlan_util.c:7:
drivers/staging/r8188eu/core/../include/wlan_bssdef.h:64:13: note: while referencing ‘data’
   64 |         u8  data[1];
      |             ^~~~

by transforming one-element array into a flexible-array member in
struct ndis_802_11_var_ie

This is part of the ongoing efforts to globally enable -Warray-bounds.
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20220202070906.GA274379@embeddedorSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/r8188eu/include/wlan_bssdef.h

@@ -61,7 +61,7 @@ struct ndis_802_11_fixed_ie {
 struct ndis_802_11_var_ie {
 	u8  ElementID;
 	u8  Length;
-	u8  data[1];
+	u8  data[];
 };
 
 /*