diag: Set the usb connected flag correctly

Diag driver sets the flag for usb connection irrespective of the
different logging modes. This patch sets the usb connection flag
only when Diag is streaming in USB mode.

Change-Id: I23f66c09242ca74b38f06700f21398f748856b41
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>