uchcom(4): validate baud rate in uchcom_pre_param

[kevlo]

Summary

Reject an invalid rate in uchcom_pre_param(), so it never reaches
uchcom_calc_baudrate(), where a rate of 0 is used as a divisor and
panics the kernel. This is what the pre_param callback is for.

This is a better approach than the previous one committed in 228b1a011660.

[aokblast]

Check uchcom code again. I think it is the correct way to detect 0 divident. pre_param is the verifier. So LGTM to me!