msm: smd_pkt: expose module parameter for the loopback edge

Newer targets have more processors than just apps and modem with SMD.
During target bring up, these other processors often have loopback channels
opened to apps for verification and stability testing.  The edge upon which
the loopback test operates is hardcoded to the apps <-> modem edge.

Currently, testing other edges with the loopback test involves editing the
kernel source, recompiling, and reloading the build onto target, which is
often a lengthy process during target bringup.  To save time, add a module
parameter which allows the user to change the loopback test edge at
runtime.

Change-Id: I2e0e07fae0cabd8fb0203d25aba4ec438f383268
Signed-off-by: Jeffrey Hugo <jhugo@codeaurora.org>