clocksource/drivers/cadence_ttc: fix a return value in case of error

IS_ERR and PTR_ERR should use the same variable, clk_ce in this case.

Fixes: 4de1eb07c47f (Convert init function to return error)

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Sören Brinkmann <soren.brinkmann@xilinx.com>