diag: Remove invalid check for payload

Call back data type does support commands which is less than
4 bytes in size. This patch removes the unnecessary check for
the call back data type.

Change-Id: Ie70196d8dd0b0b3d023bc2505fbb1cb3e582a1af
CRs-fixed: 623552
Signed-off-by: Katish Paran <kparan@codeaurora.org>