diag: Change maximum size of error response

Diag currently allows error responses of length 490 bytes. This is
outdated as we support 4K responses on the Apps. This patch changes
the Apps buffer size to 4K and also the limit on the error response
length.

Change-Id: I8212bf000a7493b756b5ac0e428c3b2e4be9e9c6

CRs-Fixed: 512035
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>