of: Fix RST inline emphasis warnings in of_map_id() kernel-doc

of: Fix RST inline emphasis warnings in of_map_id() kernel-doc

The @filter_np parameter descriptions in of_map_id() and of_map_msi_id()
contained the text '*filter_np' in prose. Docutils interprets a leading
'*' as the start of RST emphasis (italic), but finds no closing '*',
triggering:

  Documentation/devicetree/kernel-api:11: ./drivers/of/base.c:2134:
  WARNING: Inline emphasis start-string without end-string. [docutils]

  Documentation/devicetree/kernel-api:11: ./drivers/of/base.c:2260:
  WARNING: Inline emphasis start-string without end-string. [docutils]

Fix by wrapping '*filter_np' in double backticks (*filter_np) to
render it as an RST inline code literal, which is also the correct
kernel-doc convention for pointer expressions.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202606130111.ldC96rqf-lkp@intel.com/
Signed-off-by: Vijayanand Jitta <vijayanand.jitta@oss.qualcomm.com>
Link: https://patch.msgid.link/20260619-iommu_map_kdoc_fix-v1-1-9573e1cf30b3@oss.qualcomm.com
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>