ASoC: Intel: avs: Use struct_size for struct avs_modcfg_ext size

ASoC: Intel: avs: Use struct_size for struct avs_modcfg_ext size

The struct avs_modcfg_ext structure has a flexible array member for the
pin_fmts array, and the size should be calculated using struct_size to
prevent the potential for overflow with the allocation.

Reviewed-by: Cezary Rojewski <cezary.rojewski@intel.com>
Reviewed-by: "Amadeusz Sławiński" <amadeuszx.slawinski@linux.intel.com>
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://lore.kernel.org/r/20230303180457.2457069-1-jacob.e.keller@intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>