drivers: gpio: shell: streamline code

Streamline code in multiple places as follows:
- Remove redundant initialization of `ret` to `0`,
  as it is immediately assigned a value.
- Add `len` to store the result of `strlen(argv[ARGV_CONF])` to avoid
  multiple calls to `strlen` within the `for-loop` in `cmd_gpio_conf`.
- Merge separate `shell_print` calls by including a newline `\n`.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>