ALSA: usb/line6: remove 'defined but not used' warning

Fix W=1 warning. Variables are declared in a header file included from
multiple C files, replace by #defines as suggested by Takashi

sound/usb/line6/driver.h:70:18: warning: ‘SYSEX_EXTRA_SIZE’ defined
but not used [-Wunused-const-variable=]
   70 | static const int SYSEX_EXTRA_SIZE = sizeof(line6_midi_id) + 4;
      |                  ^~~~~~~~~~~~~~~~
sound/usb/line6/driver.h:69:18: warning: ‘SYSEX_DATA_OFS’ defined but
   not used [-Wunused-const-variable=]
   69 | static const int SYSEX_DATA_OFS = sizeof(line6_midi_id) + 3;
      |                  ^~~~~~~~~~~~~~

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200707184924.96291-1-pierre-louis.bossart@linux.intel.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>