diag: Free usb buffer's entry after removing from list

Currently, there is possibility of memory leak due to not
freeing allocated memory for usb buffer's entry after
removing it from list. The patch handle this by freeing
the entry.

Change-Id: Idb08ecad859749e6ab1b09184362de38de4a9836
Signed-off-by: Hardik Arya <harya@codeaurora.org>