ath10k: add missing sanity check on diag download

ath10k_hw_diag_fast_download() generally has good boundary checking, but
it misses verifying that the next metadata header actually fits in the
remaining buffer space. Add such a check.

Fixes: 39501ea6

 ("ath10k: download firmware via diag Copy Engine for QCA6174 and QCA9377.")
Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>