usb: mdm_data_bridge: Add tx/rx number of bytes counters

Add tx/rx number of bytes counters to keep track of the
total tx/rx data. This is very useful debug information.
Update the to_modem and tx_num_of_bytes counters on
data_bridge_write_cb function when urb status is success.

Change-Id: Id792ed9fd0dfc5e18911172d8a33d4e9c75c2ae4
Signed-off-by: Guang Jian Zeng <gzeng@codeaurora.org>