soc: qcom: glink_smd_xprt: add pkt_size debug logging

The SMD packet size is the size remaining in the current packet and
depending upon the size of a packet being sent, not all of the packet
may be available to read when the SMD_EVENT_DATA event has been received.

Rename pkt_size to pkt_remaining to make it clear what it contains and
add debugging statements to aid in debugging partial packet fragments.

Change-Id: Ifcfa88e8f7badca553d148fb2485bf43c5f6bd34
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>