usb: diag_bridge: Add debugfs byte counters

Add a debugfs entry to display the number of bytes transfered
between the HSIC host and the MDM DIAG interface to aid in debugging.

The debugfs entry is created at /sys/kernel/debug/diag_bridge/status.
Reading it will display the counters, while writing to it will reset.

Change-Id: I325c3af0a0490cc4a628d87b81db223c945834c5
Signed-off-by: Jack Pham <jackp@codeaurora.org>