diag: fix partial smd read

DIAG from peripheral processors sends data to Linux
using SMD channels. Sometimes data can be bigger than
the size that SMD can accommodate. This results in partial
packet. This change adds capability to read the entire
packet.

Change-Id: I5588115998ad290ab62113eea92c6b2d06931773
Signed-off-by: Shalabh Jain <shalabhj@codeaurora.org>