usb: u_data_bridge: Improve USB RX (read) functionality

This change handles below cases and improves performance:
1. If USB OUT request buffer is bigger than provided read buffer
from application, then data equal to read buffer is copied. This
results into data loss. Handle this case by making sure that
whole USB OUT request buffer is provided to application across
multiple read() call.

2. If read buffer size is greater than available data as part of
one USB OUT request buffer, then copy data from other USB OUT
request buffers until read buffer is filled completely.

Change-Id: If2756ed82bd6e95650ebcc38c4042bfb593f8a2b
Signed-off-by: Mayank Rana <mrana@codeaurora.org>