clk: stm32f4: fix error handling

This is likely that checking 'clks[idx]' instead of 'clks[n]' is
expected here.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Link: lkml.kernel.org/r/1467526003-13318-1-git-send-email-christophe.jaillet@wanadoo.fr