BTReal: Treat libusb_get_string_descriptor_ascii failure as non-fatal warning

[Pokechu22]

See https://forums.dolphin-emu.org/Thread-unable-to-pair-remotes-hci-command-timeout, which was apparently introduced by #10729.

My guess as to what's happening here is that device_descriptor.iSerialNumber is 0, and libusb_get_string_descriptor_ascii doesn't like 0 as it indicates a special case, returning LIBUSB_ERROR_INVALID_PARAM in that case (which matches the log). However, I haven't tested this change at all.

[AdmiralCurtiss]

Looks reasonable. I've asked the person on the forums to check if this fixes it, let's see what they say.

[AdmiralCurtiss]

This improved the situation but they're still having problems btw: https://forums.dolphin-emu.org/Thread-unable-to-pair-remotes-hci-command-timeout?pid=526505#pid526505

Anything in that log that sticks out for you? I'm not seeing anything obvious.

[Pokechu22]

No, I don't see anything obvious either. It might be useful to compare to a log from a known-working version (and to make sure that pairing multiple still works in that old version), though.