soc: qcom: glink_smd_xprt: rename data and control references

The SMD Transitional transport maintains a single control channel to
each remote subsystem in addition to multiple data channels, but it is
sometimes difficult through code inspection to confirm which channel is
being operate on leading to temporary confusion.

Rename functions and variables for the control and data channels to make
the code easier to read.

Change-Id: If0aa370cdce8fc5be25badcd1a33111aa8228c19
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>