ftdi-la: do enter the error path upon VID:PID mismatch

Bug #1390 reports that "!desc" is always true (should be: false?). But
the actual problem would be that 'desc' is _not_ NULL when none of the
supported chips' VID:PID matched (FT232H happens to "get found" then,
erroneously).

Add a sentinel to the table of supported chips, such that 'desc' becomes
NULL upon mismatch, and the error path is entered.