net: wifi: shell: streamlined local variable initializations

This commit simplifies the access to structure members and omits
unnecessary variable initializations.

Specific adjustments include:
- Moving from pointer-based access (`(&regd)->chan_info`) to
  direct structure member access (`regd.chan_info`).
- Removing explicit initializations where not required.
- Removing excess backslashes '\' before '%' in the format string.

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