mbedtls: avoid function that only exists on rp2040

[jepler]

rp2350 doesn't have the same RTC peripheral as the 2040 (let alone other family micros), as reported on discord by anecdata.

this has the side effect of obeying a different timesource if one has been configured.

This only compile-tested (for pico w and for pico 2 with SSL module enabled). I didn't actually run the code.

@anecdata please test if you can.

[anecdata]

Tested Requests on:

• Pico W, SSL over wifi
• WIZnet W5500 EVB Pico, SSL over Ethernet
• Pico2 (WIZnet W5100S EVB Pico2 - RP2350), but without SSL, over Ethernet

Looks good to me, thank you!

(I'll test Pico2 with SSL over Ethernet on my WIZnet W5100S EVB Pico2 PR #9791)

edit: not sure what "obeying a different timesource if one has been configured" implies but time.localtime() behaves as expected when used with default CPU time and with RTC source set to NTP:

print(f'{time.localtime()}')
ntp = adafruit_ntp.NTP(pool)
rtc.set_time_source(ntp)
print(f'{time.localtime()}')

after a reset yields:

struct_time(tm_year=2000, tm_mon=1, tm_mday=1, tm_hour=0, tm_min=0, tm_sec=12, tm_wday=5, tm_yday=1, tm_isdst=-1)
struct_time(tm_year=2024, tm_mon=11, tm_mday=5, tm_hour=2, tm_min=15, tm_sec=42, tm_wday=1, tm_yday=310, tm_isdst=-1)

[dhalbert]

Thanks for the fix and the review!