clk: gpio: Unlock mutex on error path

We don't unlock the mutex if we fail to allocate the parent names
array. Unlock it and return an error in this case as well.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Acked-by: Julia Lawall <julia.lawall@lip6.fr>
Cc: Sergej Sawazki <ce3a@gmx.de>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>