USB: gadget: f_audio_source: change max ISO packet size

Change it to 256 bytes from 384 bytes.

Most USB-AUDIO devices are limited to 256 byte for max iso buffer size. If a
IN_EP_MAX_PACKET_SIZE is bigger than a USB-AUDIO device's max iso buffer size,
it will cause noise. This patch will prevent this case as possibe by reducing
packet size.  When using 44.1khz, 2ch, 16bit audio data, if max packet size is
bigger than 176 bytes, it's no problem.

Signed-off-by: Iliyan Malchev <malchev@google.com>