rpmsg: glink: Use spinlock in tx path

Switch the tx_lock to a spinlock we allow clients to use rpmsg_trysend()
from atomic context.

In order to allow clients to sleep while waiting for space in the FIFO
we release the lock temporarily around the delay; which should be
replaced by sending a READ_NOTIF and waiting for the remote to signal
us that space has been made available.

Change-Id: Id3f3f7bed80ca9db9942210bf2cd5cfa3b8f3d3e
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Patch-mainline: linux-arm-msm @ 13/02/2018 11:04
[clew@codeaurora.org: Fix uninitialized return value in qcom_glink_tx]
Signed-off-by: Chris Lew <clew@codeaurora.org>