ASoC: rt5645: Constify ACPI device ids

Constify the ACPI device ID array, no need to have it writable at
runtime. Also drop the unused RT5645_INIT_REG_LEN define.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Cc: Bard Liao <bardliao@realtek.com>
Cc: Oder Chiou <oder_chiou@realtek.com>
Cc: John Lin <john.lin@realtek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>