msm: smd: Handle corrupted packet header gracefully

SMD library doesn't report error when the SMD packet is of invalid
length(length > 0x7fffffff). When a SMD user(for example SMD_PKT) tries
to read the packet size using an "int" variable the value of packet size
becomes negative resulting into unintended behavior.

Validate the size of the recieved SMD packet.

CRs-Fixed: 579605
Change-Id: Icb12ae6bbb29104f4039209ba142120697271b3a
Signed-off-by: Zaheerulla Meer <zmeer@codeaurora.org>