ALSA: usb: fcp: Fix return code from poll ops

Fix a sparse warning due to the invalid return type from poll ops,
which is __poll_t.

Fixes: 46757a3e ("ALSA: FCP: Add Focusrite Control Protocol driver")
Link: https://patch.msgid.link/20250121170032.7236-4-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>