net: wwan: mhi_wwan_mbim: Avoid -Wflex-array-member-not-at-end warning

net: wwan: mhi_wwan_mbim: Avoid -Wflex-array-member-not-at-end warning

Use DEFINE_RAW_FLEX() to avoid a -Wflex-array-member-not-at-end warning.

Remove fixed-size array struct usb_cdc_ncm_dpe16 dpe16[2]; from struct
mbim_tx_hdr, so that flex-array member struct mbim_tx_hdr::ndp16.dpe16[]
ends last in this structure.

Compensate for this by using the DEFINE_RAW_FLEX() helper to declare the
on-stack struct instance that contains struct usb_cdc_ncm_ndp16 as a
member. Adjust the rest of the code, accordingly.

So, with these changes fix the following warning:

drivers/net/wwan/mhi_wwan_mbim.c:81:34: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Reviewed-by: Loic Poulain <loic.poulain@oss.qualcomm.com>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>