net: l2: wifi: wifi_utils: Resolve build warning with strncpy function

ARM GCC version 12.2.0 (Zephyr SDK 0.16.4) generates the following build
warning from the strncpy call in "wifi_utils_parse_scan_bands":

warning: '__builtin_strncpy' output truncated before terminating nul
copying as many bytes from a string as its length

To resolve this warning, pass the maximum length of the temporary
parse_str buffer to strncpy. This also has the benefit of correctly null
terminating parse_str, since we already verify the
scan_bands_str is properly null terminated with the strlen() check in
this function. We can therefore remove the line adding a null terminator
to parse_str as well.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>