coresight: Fix incorrect handling for return value of devm_kzalloc

The return value of devm_kzalloc could be an null pointer,
use "!desc.pdata" to fix incorrect handling return value
of devm_kzalloc.

Fixes: 4277f035 ("coresight: trbe: Add a representative coresight_platform_data for TRBE")
Signed-off-by: Lin Yujun <linyujun809@h-partners.com>
Reviewed-by: James Clark <james.clark@linaro.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20250908122022.1315399-1-linyujun809@h-partners.com