rpmsg: smd: Use spinlock in tx path

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

The mutex was interruptable as it was previously held for the duration
of some client waiting for available space in the FIFO, but this was
recently changed to only be held temporarily - allowing us to replace it
with a spinlock.

Tested-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>