clk: at91: Fix a return value in case of error

If 'clk_hw_register()' fails, it is likely that we expect to return an
error instead of a valid pointer (which would mean success).

Fix commit f5644f10 ("clk: at91: Migrate to clk_hw based registration
and OF APIs")

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>