diag: dci: Remove the cap on number of delayed rsp registrations

Currently, diag driver allows a definite number of command
registrations. This limit can be reached easily since we don't
delete delayed rsp cmd registrations. Make changes to allow
any number of cmd-rsp registrations and to remove delayed rsp
cmd registrations after receiving its corresponding last rsp.

Change-Id: Ia62bb3cf8a1359568c2579acee490ec2b246ee75
CRs-Fixed: 583922
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>