scripts: west: runners: stm32cubeprogrammer: fix E0606 related to dl_file

Initialize 'dl_file' to 'None' before the logic that determines
which binary file to load.

Resolves the E0606 Pylint warning.

Signed-off-by: Georgij Černyšiov <geo.cgv@gmail.com>

scripts/west_commands/runners/stm32cubeprogrammer.py

@@ -270,6 +270,8 @@ class STM32CubeProgrammerBinaryRunner(ZephyrBinaryRunner):
             self.check_call(cmd + ["--erase", "all"])
 
         # Define binary to be loaded
+        dl_file = None
+
         if self._use_elf:
             # Use elf file if instructed to do so.
             dl_file = self.cfg.elf_file