pcm: hw: fix to initialize function local variable

pcm: hw: fix to initialize function local variable

This commit is to fix below warning.

pcm_hw.c: In function ‘snd1_pcm_hw_open_fd’:
pcm_hw.c:955:33: warning: ‘mmap_control’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  if (mmap_control == MAP_FAILED || mmap_control == NULL) {
                                 ^
pcm_hw.c:946:31: note: ‘mmap_control’ was declared here
  struct snd_pcm_mmap_control *mmap_control;
                               ^~~~~~~~~~~~

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>