firmware: stratix10-svc: release mutex sdm_lock after processing command

When loading intel-fcs.ko kernel module, system hangs up. This issue
is introduced by sdk commit fc23818c8dfa("HSD #14015013554: firmware:
stratix10-svc: support up to 4 SVC clients "). This commit intends to declear
that stratix10-svc driver can support 4 SVC clients by allocating their own
channel to communicate with ATF, and use a  mutex to control thread
synchronization. But the mutex sdm_lock is not released after processing
command, and cause a deadlock issue. So, add the mutex release code after
processing command.

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