modem/msm-modem: rename msm-modem-mainline to msm-modem-rpmsg

Newer Qualcomm SoCs (e.g. SDM845. MSM8953) seem to use a different method to talk to the modem. There are no longer separate rpmsg/SMD channels for QMI messags, instead the modem is also available through QRTR.

On these newer SoCs, installing msm-modem-mainline is pointless, because the created /dev/modem device will not actually allow communicating with the modem. However, you still need the part from the base package (msm-modem), which installs+enables rmtfs.

To avoid confusion, rename the msm-modem-mainline subpackage to msm-modem-rpmsg. Install only "msm-modem" on SDM845 because that likely does not allow communication with the modem through RPMSG anyway.

Cc: @kalube