ALSA: fm801: put curly braces around empty if-body

The compiler complains on unused condition as follows

sound/pci/fm801.c: In function ‘snd_fm801_interrupt’:
sound/pci/fm801.c:585:3: warning: suggest braces around empty body in an ‘if’ statement [-Wempty-body]

Put the curly braces around empty body as suggested.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>