net: qualcomm: rmnet: flush uplink aggregation on priority tx

If the RmNet driver needs to transmit a packet marked as needing
prioritization, it should skip uplink aggregation of the packet to avoid
adding additional delay. Additionally, any aggregated packets should be
flushed prior to sending the prioritized packet to avoid any ordering
issues.

Change-Id: Ia3063ec3f0553693a0f66c8eb5bbb88d250f7523
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>