ath10k: fix ath10k_pci struct layout

ath10k: fix ath10k_pci struct layout

gcc-10 correctly points out a bug with a zero-length array in
struct ath10k_pci:

drivers/net/wireless/ath/ath10k/ahb.c: In function 'ath10k_ahb_remove':
drivers/net/wireless/ath/ath10k/ahb.c:30:9: error: array subscript 0 is outside the bounds of an interior zero-length array 'struct ath10k_ahb[0]' [-Werror=zero-length-bounds]
   30 |  return &((struct ath10k_pci *)ar->drv_priv)->ahb[0];
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from drivers/net/wireless/ath/ath10k/ahb.c:13:
drivers/net/wireless/ath/ath10k/pci.h:185:20: note: while referencing 'ahb'
  185 |  struct ath10k_ahb ahb[0];
      |                    ^~~

The last addition to the struct ignored the comments and added
new members behind the array that must remain last.

Change it to a flexible-array member and move it last again to
make it work correctly, prevent the same thing from happening
again (all compilers warn about flexible-array members in the
middle of a struct) and get it to build without warnings.

Fixes: 521fc37be3d8 ("ath10k: Avoid override CE5 configuration for QCA99X0 chipsets")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200509120707.188595-2-arnd@arndb.de