pcm: rate: Fix compile warning wrt bit ops and comparison

pcm: rate: Fix compile warning wrt bit ops and comparison

We've got a gcc warning:
  pcm_rate.c: In function ‘snd_pcm_rate_drain’:
  pcm_rate.c:1090:19: warning: suggest parentheses around comparison in operand of ‘&’ [-Wparentheses]
     if (pcm->mode & SND_PCM_NONBLOCK != 0) {
                   ^

Drop the zero comparison for fixing the warning and for simplicity.

Fixes: 29041c522071 ("fix infinite draining of the rate plugin in SND_PCM_NONBLOCK mode")
Signed-off-by: Takashi Iwai <tiwai@suse.de>