cam: fix sign-extension error in adagetparams

adagetparams contains a sign-extension error that will cause the sector
count to be incorrectly calculated for ATA disks of >=1TiB that still use
CHS addressing. Disks using LBA48 addressing are unaffected.

Reported by: Coverity
CID: 1007296
Reviewed by: ken
MFC after: 3 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D13198