clk: at91: fix check of clk_register() returned value

The clk_register() function returns a valid pointer to struct clk or
ERR_PTR() error code, this makes a check for returned NULL value
useless and may lead to oops on error path.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Fixes: bcc5fd49 ("clk: at91: add a driver for the h32mx clock")
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>