net: wifi_shell: Add user input validation for SSID and PSK

When parsing user input for "wifi connect" and "wifi ap enable"
commands, the SSID and PSK lengths were not verified. It's better to
detect invalid connect/AP enable parameters early, so that help text can
be printed, instead of letting wifi_mgmt command to fail.

For WIFI_SECURITY_TYPE_SAE, follow the Linux convention of limiting the
size to 128 bytes.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>