clk: x86: pmc-atom: Checking for IS_ERR() instead of NULL

clkdev_hw_create() returns NULLs on error, it doesn't return error
pointers.

Fixes: 41ee7caf

 ("clk: x86: add "mclk" alias for Baytrail/Cherrytrail")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>