diag: Remove unsupported smd control channel code

The diag smd control channel implementation only supports
a single data buffer for processing data. Remove stale code
that tries to access a second buffer.

CRs-Fixed: 521621
Change-Id: I9dc0224d9f4d0b9cb0a836cf2bd33459c15cc2a7
Signed-off-by: Dixon Peterson <dixonp@codeaurora.org>