ASoC: Intel: avs: Simplify avs_get_i2s_boards()

No functional changes, just code simplification and readability
improvements as there is no need to operate on 'struct avs_acpi_boards'
if 'struct snd_soc_acpi_mach' is what interests us.

Reviewed-by: Amadeusz Sławiński <amadeuszx.slawinski@linux.intel.com>
Signed-off-by: Cezary Rojewski <cezary.rojewski@intel.com>
Message-ID: <20250827142229.869139-5-cezary.rojewski@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>