clk: mvebu: armada-37xx-periph: Fix wrong return value in get_parent

The return value of the get_parent operation is a u8, whereas a -EINVAL
was returned. This wrong value was return if the value was bigger that
the number of parent but this case was already handled by the core.

So we can just remove this chunk of code to fix the issue.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 9818a7a4 ("clk: mvebu: armada-37xx-periph: prepare cpu clk to
be used with DVFS")
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>