uart_dev_snps: Fix device probing

The "uart_bus_probe" function is used as a generic part of uart probe
logic. It returns a driver priority(negative number) if successful and
an error code otherwise.
Fix the error checking condition to account for that.
Also, while here return "BUS_PROBE_VENDOR", instead of "0".
This fixes uart on clearfog pro with recent DT.

PR: 266657
Reviewed by: mw
Obtained from: Semihalf
Differential Revision: https://reviews.freebsd.org/D36880