wil6210: increase cmd buffer size to avoid sscanf buffer overflow

cppcheck detected a buffer overflow:

[drivers/net/wireless/ath/wil6210/debugfs.c:634]: (error) Width 8
  given in format string (no. 1) is larger than destination buffer
  'cmd[8]', use %7s to prevent overflowing it.

For the current %8s sscanf we require cmd to be 9 chars long
so increase it by 1 byte to prevent the sscan overflow (rather
than reduce the %8s specifier to %7s as cppcheck recommends).

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>