wil6210: avoid gcc-10 zero-length-bounds warning

gcc-10 warns about accesses inside of a zero-length array:

drivers/net/wireless/ath/wil6210/cfg80211.c: In function 'wil_cfg80211_scan':
drivers/net/wireless/ath/wil6210/cfg80211.c:970:23: error: array subscript 255 is outside the bounds of an interior zero-length array 'struct <anonymous>[0]' [-Werror=zero-length-bounds]
  970 |   cmd.cmd.channel_list[cmd.cmd.num_channels++].channel = ch - 1;
      |   ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
In file included from drivers/net/wireless/ath/wil6210/wil6210.h:17,
                 from drivers/net/wireless/ath/wil6210/cfg80211.c:11:
drivers/net/wireless/ath/wil6210/wmi.h:477:4: note: while referencing 'channel_list'
  477 |  } channel_list[0];
      |    ^~~~~~~~~~~~

Turn this into a flexible array to avoid the warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200505143332.1398524-1-arnd@arndb.de