diag: Synchronize MHI read and USB write for diag buffers

Remove the buffer entry from the usb table in error cases and
add support for knowing USB connection status. Remove the buffer
entry from the mhi callback list if entry is removed from mhi
buffer table as well.
Increase the scope of channel lock while memory allocation and
synchronize it against memory free.

Change-Id: If22cef15898fc8a5ec727342965b597a0a256016
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>