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

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
Link: https://github.com/KSPP/linux/issues/202
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/ZgHYE2s5kBGlv1cw@neat
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>