char: xilinx_hwicap: Fix NULL vs IS_ERR() bug

The devm_platform_ioremap_resource() function returns error pointers.
It never returns NULL.  Update the check accordingly.

Fixes: 67237183

 ("char: xilinx_hwicap: Modernize driver probe")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Acked-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/ef647a9c-b1b7-4338-9bc0-28165ec2a367@moroto.mountain
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>