remove heuristic error detection from ddi_strto*()

Zero, <TYPE>_MIN and <TYPE>_MAX values can result from valid conversions.
They don't necessarily imply any error.
Since we do not have any reliable error signaling from libkern's strto*(),
it's better to always assume success rather than to report an error when
there is none.

Reviewed by: tsoome
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D12565