diag: Fix parsing of smd control channel buffer

Fix parsing of data sent on the smd control channel to
accomodate types of data other than DIAG_CNTRL_MSG_REG.
Currently, if the type of data in the buffer is other than
DIAG_CNTRL_MSG_REG, progression through the buffer is
not performed correctly.

CRs-fixed: 334307
Change-Id: I406a437168956277190d2bcf2e94345d2eec0783
Signed-off-by: Dixon Peterson <dixonp@codeaurora.org>