rpmsg: glink: Consolidate TX_DATA and TX_DATA_CONT

Rather than duplicating most of the code for constructing the initial
TX_DATA and subsequent TX_DATA_CONT packets, roll them into a single
loop.

Signed-off-by: Bjorn Andersson <quic_bjorande@quicinc.com>
Reviewed-by: Chris Lew <quic_clew@quicinc.com>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230418163018.785524-3-quic_bjorande@quicinc.com