net: qrtr: Implement outgoing flow control

In order to prevent overconsumption of resources on the remote side QRTR
implements a flow control mechanism.

The mechanism works by the sender keeping track of the number of
outstanding unconfirmed messages that has been transmitted to a
particular node/port pair.

Upon count reaching a low watermark (L) the confirm_rx bit is set in the
outgoing message and when the count reaching a high watermark (H)
transmission will be blocked upon the reception of a resume_tx message
from the remote, that resets the counter to 0.

This guarantees that there will be at most 2H - L messages in flight.
Values chosen for L and H are 5 and 10 respectively.

Change-Id: Ifa3c0c5821d31e385c34428d0bd4f5b5eec2fb77
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Patch-mainline: linux-arm-kernel @ 07/05/19, 23:06
[clew@codeaurora.org: Fix uninitialized qrtr_tx_lock mutex]
Signed-off-by: Chris Lew <clew@codeaurora.org>