-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]