usb: gadget: f_audio_source: change max ISO packet size

Re-applying from
https://gitorious.org/shr/linux/commit/eb4c9d2db894c3492c0a848581bd4f6790f93d5f

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.

Credits to: Iliyan Malchev <malchev@google.com>

Change-Id: Ic2a1c19ea65d5fb42bf12926b51b255b465d7215
Signed-off-by: Anson Jacob <ansonkuzhumbil@gmail.com>