ALSA: pci/emu10k1: remove 'set but not used' warning

Fix W=1 warning. The loopsize variable is only used in compiled-out
code, so mark with __maybe_unused.

sound/pci/emu10k1/emu10k1_patch.c: In function
‘snd_emu10k1_sample_new’:
sound/pci/emu10k1/emu10k1_patch.c:30:22: warning: variable ‘loopsize’
set but not used [-Wunused-but-set-variable]
   30 |  int truesize, size, loopsize, blocksize;
      |                      ^~~~~~~~

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200702193604.169059-14-pierre-louis.bossart@linux.intel.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>