rpmsg: glink: spi: Use irq safe spinlocks

The glink spi command handler is done from the wcd spi component
context which is not necessarily a true interrupt. Use the irqsave and
irqrestore APIs to avoid any spinlock issues.

Change-Id: Id1857d44f8bf2da4550ece30284dd6890851e686
Signed-off-by: Chris Lew <clew@codeaurora.org>