Coresight: Fix a NULL vs IS_ERR() bug in probe

The devm_platform_get_and_ioremap_resource() function doesn't
return NULL, it returns error pointers.  Update the checking to
match.

Fixes: f78d206f ("Coresight: Add Coresight TMC Control Unit driver")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/dab039b9-d58a-41be-92f0-ff209cfabfe2@stanley.mountain