Add readability-identifier-length to tolerate 2-length variable names…

… which is regularly used in the library (new warning/error due to clang-tidy version update)

Signed-off-by: Sébastien LAIGRE <slaigre@silicom.fr>

.clang-tidy

@@ -22,6 +22,7 @@ Checks: '-*,
   -readability-magic-numbers,
   -readability-redundant-declaration,
   -readability-redundant-access-specifiers,
+  -readability-identifier-length
 '
 
 WarningsAsErrors: '*'