diag: Poll SMD control channel for data after USB connects

DIAG uses SMD control channels to transfer packet registration
information from slave processors. While USB is disconnected, under
some scenarios, the SMD data might be left unread. Hence, when USB
gets connected, a special effort has to be made to read any unread
data on SMD control channel. Also each time a registration information
is stored in diag table, the SMD channel needs to be polled once more.

Signed-off-by: Shalabh Jain <shalabhj@codeaurora.org>