checkpatch: ignore COMPARISON_TO_NULL

Ignore COMPARISON_TO_NULL since it conflicts with Zephyr coding
guidelines main rule 85:
  The controlling expression of an if statement and the
  controlling expression of an iteration-statement shall have
  essentially Boolean type

Link: https://docs.zephyrproject.org/latest/contribute/coding_guidelines
Signed-off-by: Etienne Carriere <etienne.carriere@st.com>

.checkpatch.conf

@@ -29,3 +29,4 @@
 --ignore ENOSYS
 --ignore IS_ENABLED_CONFIG
 --ignore EXPORT_SYMBOL
+--ignore COMPARISON_TO_NULL