ALSA: es1688: Remove set but not used variable 'hw'

Fixes gcc '-Wunused-but-set-variable' warning:

sound/isa/es1688/es1688_lib.c: In function 'snd_es1688_probe':
sound/isa/es1688/es1688_lib.c:124:31: warning:
 variable 'hw' set but not used [-Wunused-but-set-variable]
  unsigned short major, minor, hw;
                               ^

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>