WiFi Monitor: improve channel validation

[anecdata]

Test code:

import wifi

# new check in monitor.channel
monitor = wifi.Monitor()
for channel in range(-1, 16):
    try:
        monitor.channel = channel
        print(monitor.channel, "channel OK")
    except ValueError as e:
        print(channel, e)

print()

# corrected check in wifi.Monitor(channel=)
for channel in range(-1, 16):
    try:
        monitor = wifi.Monitor(channel=channel)
        print(monitor.channel, "channel OK")
        monitor.deinit()
    except ValueError as e:
        print(channel, e)

Result (same for both loops):

code.py output:
-1 channel out of bounds
0 channel out of bounds
1 channel OK
2 channel OK
3 channel OK
4 channel OK
5 channel OK
6 channel OK
7 channel OK
8 channel OK
9 channel OK
10 channel OK
11 channel OK
12 channel OK
13 channel OK
14 channel out of bounds
15 channel out of bounds

[tannewt]

Looks good! Thank you!