ALSA: firewire-motu: mark trace helpers as __maybe_unused

ALSA: firewire-motu: mark trace helpers as __maybe_unused

Two functions were introduced for the purpose of tracing but cause warnings
when tracing is disabled:

sound/firewire/motu/amdtp-motu.c:284:13: error: 'copy_message' defined but not used [-Werror=unused-function]
 static void copy_message(u64 *frames, __be32 *buffer, unsigned int data_blocks,
sound/firewire/motu/amdtp-motu.c:271:13: error: 'copy_sph' defined but not used [-Werror=unused-function]
 static void copy_sph(u32 *frames, __be32 *buffer, unsigned int data_blocks,

Marking them as __maybe_unused will do the right thing here.

Fixes: 17909c1b3058 ("ALSA: firewire-motu: add tracepoints for SPH in IEC 61883-1 fashion")
Fixes: c6b0b9e65f09 ("ALSA: firewire-motu: add tracepoints for messages for unique protocol")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>