net: ipc_router: Remove header length check

V1 header length check for read_avail cause read failures if packet
comes with V2 header and payload less than 16bytes.

Remove the V1 header length check to read small packets once it receives.

CRs-Fixed: 2279076
Change-Id: I7daf5c150e501d209057d6f2af998f685f5cd25d
Signed-off-by: Arun Kumar Neelakantam <aneela@codeaurora.org>