rpmsg: glink: spi: Reduce tx sleep

The glink spi use case have strict latency requirements. Reduce the tx
wait time to improve latency during high throughput usecases.

Change-Id: Ief636ecd0def50ab59df1f190a7dd735e89c8344
Signed-off-by: Chris Lew <clew@codeaurora.org>