staging: rtl8723bs: Allocate full pwep structure

The pwep allocation was always being allocated smaller than the true
structure size. Avoid this by always allocating the full structure.
Found with GCC 12 and -Warray-bounds:

../drivers/staging/rtl8723bs/os_dep/ioctl_linux.c: In function 'rtw_set_encryption':
../drivers/staging/rtl8723bs/os_dep/ioctl_linux.c:591:29: warning: array subscript 'struct ndis_802_11_wep[0]' is partly outside array bounds of 'void[25]' [-Warray-bounds]
  591 |                         pwep->length = wep_total_len;
      |                             ^~

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Fabio Aiuto <fabioaiuto83@gmail.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: linux-staging@lists.linux.dev
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220608215512.1070847-1-keescook@chromium.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>