util_macros.h: fix the reference in kernel-doc

In PTR_IF() description the text refers to the parameter as (ptr) while
the kernel-doc format asks for @ptr.  Fix this accordingly.

Link: https://lkml.kernel.org/r/20250428072737.3265239-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Alexandru Ardelean <aardelean@baylibre.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>