diag: Reset the in_busy variable on closing the socket

Reset the in_busy variable for the buffer that reads the
data from socket channel when diag socket state is closed.

Change-Id: Ib24ce7c58397709b7199f7656d251976a1d2c4ba
Signed-off-by: Sreelakshmi Gownipalli <sgownipa@codeaurora.org>