clk: mxl: syscon_node_to_regmap() returns error pointers

Commit 03617731 ("clk: mxl: Switch from direct readl/writel based IO
to regmap based IO") introduced code resulting in below warning issued
by the smatch static checker.

  drivers/clk/x86/clk-lgm.c:441 lgm_cgu_probe() warn: passing zero to 'PTR_ERR'

Fix the warning by replacing incorrect IS_ERR_OR_NULL() with IS_ERR().

Fixes: 03617731 ("clk: mxl: Switch from direct readl/writel based IO to regmap based IO")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Rahul Tanwar <rtanwar@maxlinear.com>
Link: https://lore.kernel.org/r/49e339d4739e4ae4c92b00c1b2918af0755d4122.1666695221.git.rtanwar@maxlinear.com


Signed-off-by: Stephen Boyd <sboyd@kernel.org>