driver: firmware: stratix10-svc: schedule thread out when there is no data reveived

In thread svc_normal_to_secure_thread(), function kfifo_out_spinlocked()
always return, so this thread can't release cpu even if there is no data
received, and cause cpu is under heave load status. System performance
is poor.

This issue is introduced by commit 7ca5ce89("firmware: add Intel
Stratix10 service layer driver")

Therefore, schedule this thread out when there is no data reveived, and
wake it up after sending data to it.

Signed-off-by: Meng Li <Meng.Li@windriver.com>