[ALSA] usb-audio: don't use empty packets at start of playback

Modules: USB generic driver

Some widespread USB interface chips with adaptive iso endpoints hang
after receiving a series of empty packets when they expect data (ALSA
bug#1585).  To avoid this, we have to send packets containing silence
(zero samples) instead.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>