drivers: net: rmnet: Priority callbacks

Introduce the concept of priority for dl
marker callbacks. This ensures that
clients are allowed and able to flush
before downstream clients when we know
a modem burst has finished.

Change-Id: Ic2049d7557af4da291d11a19601afe678555b286
Signed-off-by: Conner Huff <chuff@codeaurora.org>