usb: u_data_bridge: Fix handling of USB RX requests

Currently on USB composition switch or USB cable disconnect,
USB RX requests are being added as part of read_queued list
instead of read_pool. In some cases it is seen that if port
bridge is trying to read data, those requests are requeued
and fails with -ESHUTDOWN, and again it continues with results
into watchdog timeout due to error log coming from read request
completion handler. Fix this by adding proper check in read
request completion handler.

Also reduce USB Tx/RX buffer size from PAGE_SIZE to 2048 bytes.
It also fixes issue where USB RX requests are not being queued
after USB composition switch.

CRs-Fixed: 854301
Change-Id: I3616dc35c093cfa19865891f46e7b1ccc7302573
Signed-off-by: Mayank Rana <mrana@codeaurora.org>