usb: gadget: f_cdev: increase read buffer size

Increasing the read buffer size from 2k to 50k.
changes to read single buffer at a time.

Change-Id: I3d6483b809fddf6032b6dbb7f0edba0ce30c7d02
Signed-off-by: Aditya Raut <aditraut@codeaurora.org>