thinkpad_acpi: Mark volume_alsa_control_{vol,mute} as __initdata

Mark volume_alsa_control_vol and volume_alsa_control_mute as __initdata,
as snd_ctl_new1() will copy the relevant parts, so there is no need to
keep the master copies around after initialization.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Cc: Henrique de Moraes Holschuh <ibm-acpi@hmh.eng.br>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>