clk: fix a panic error caused by accessing NULL pointer

clk: fix a panic error caused by accessing NULL pointer

[ Upstream commit 975b820b6836b6b6c42fb84cd2e772e2b41bca67 ]

In some cases the clock parent would be set NULL when doing re-parent,
it will cause a NULL pointer accessing if clk_set trace event is
enabled.

This patch sets the parent as "none" if the input parameter is NULL.

Fixes: dfc202ead312 (clk: Add tracepoints for hardware operations)
Signed-off-by: Cai Li <cai.li@spreadtrum.com>
Signed-off-by: Chunyan Zhang <chunyan.zhang@spreadtrum.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>