mei: Avoid a bunch of -Wflex-array-member-not-at-end warnings

-Wflex-array-member-not-at-end is coming in GCC-14, and we are getting
ready to enable it globally.

After commit 40292383640a ("mei: revamp mei extension header structure layout.")
it seems that flexible-array member `data` in `struct mei_ext_hdr` is no longer
needed. So, remove it and, with that, fix 45 of the following
-Wflex-array-member-not-at-end warnings[1] in drivers/misc/mei/:

drivers/misc/mei/hw.h:280:28: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Link: https://gist.github.com/GustavoARSilva/62dcc235555a6b29b506269edb83da0b [1]
Link: https://github.com/KSPP/linux/issues/202
Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Link: https://lore.kernel.org/r/ZgHYE2s5kBGlv1cw@neat
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/misc/mei/hw.h

···
  * struct mei_ext_hdr - extend header descriptor (TLV)
  * @type: enum mei_ext_hdr_type
  * @length: length excluding descriptor
- * @data: the extended header payload
  */
 struct mei_ext_hdr {
 	u8 type;
 	u8 length;
-	u8 data[];
 } __packed;
 
 /**