usb: gadget: cdev: Add single packet and dynamic buffer support for Rx path

Display tearing functionality on SXR devices makes use of
both  bulk-in and bulk-out endpoints of cdev driver
to acquire real time audio/video data from usb host.

Currently if the userspace application making use of cdev
driver requests 50KB of data from host, the cdev_read function
queues multiple usb requests to host till it gets 50KB of data.
In Display Tearing, although the userspace application requests
50KB of data per frame, the actual amount sent by host for every
frame is variable and can be less than the requested amount. If
host is sending packets of size less that requested amount per frame,
driver ends up appending multiple frames in a single cdev_read call
and returns corrupt information to userspace causing low frame
rate on display.

To resolve this add a configfs property to specify the buffer size
for every usb request on out ep and a flag to specify if we need
single packet mode. If single packet mode is enabled, only one usb
request is queued on out endpoint per cdev_read call and buffer
corresponding to that request contains only one frame of data coming
from host. The driver appends data from one usb request in read_queued
pool to userspace buffer to avoid corrupting video frame information.

Change-Id: I66dcc57513c1cf0ec87f16bde316e1230deabcbf
Signed-off-by: Ligui Deng <ldeng@codeaurora.org>
Signed-off-by: Krishna Kurapati <quic_kriskura@quicinc.com>