net: qrtr: mhi: Create list for pending ul transfers

QRTR does not serialize sends like the dl path. Create a list of
pending transactions so multiple transfers can happen at the same time
without corruption. If there is a sock attached to the skb, take a
reference to the sock. This prevents the sock from being released while
waiting for the ul callback from MHI.

Change-Id: I2d9359ec616497f3dda4f2762d142cdd33796a88
Signed-off-by: Chris Lew <clew@codeaurora.org>