sesutil: Avoid setting reserved bits.

Weird side of SES specification is that some bits have different
meaning or semantics in status and control pages. This patch fixes
non-zero writes into reserved fields, that caused errors on some
enclosures when trying to control locate/fault LEDs, keeping other
bits unchanged.

MFC after: 2 weeks
Sposonred by: iXsystems, Inc.

(cherry picked from commit 2e19fae49fd4ab2360971463cee99a62198973b1)