net: wifi: Fix crash in wifi_utils_parse_scan_ssids

wifi_utils_parse_scan_ssids could cause a crash if a constant string is
passed to it. Fix this by duplicating the input string parameter before
parsing it with strtok_r.

Also limit the range of the CONFIG_WIFI_SCAN_SSID_FILT_MAX parameter
from 1 to 4 to avoid stack overflow due to users specifying a large
value for this parameter.

Signed-off-by: Sachin D Kulkarni <sachin.kulkarni@nordicsemi.no>