net: l2: wifi: Fix mode change issue in Wi-Fi shell

The help message for the wifi mode -s command indicates it sets
the Wi-Fi mode to station. However, the Zephyr Wi-Fi shell subsystem
was incorrectly using a get operation instead of set.

This commit corrects the behavior to properly set the Wi-Fi mode,
aligning the implementation with the help message.

Signed-off-by: Arunmani Alagarsamy <arunmani.a@silabs.com>