diag: Add flow control counts to diag-socket layer

If the peripherals are slow in reading diag data from socket
channels, the channel will be flow controlled. Add counters to
track how many times a peripheral has flow controlled Apps Diag
driver.

Change-Id: I6907cc618d50f7343efe964eeb07668cdb6e4249
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>