misc: fastrpc: use correct spinlock variant

context spin lock can be interrupted from callback path so use correct spinlock
so that we do not hit spinlock recursion.

Fixes: c68cfb71 ("misc: fastrpc: Add support for context Invoke method")
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>