jedec_dimm(4): Refactor offset adjustment and page0 reset

Offsets greater than 255 bytes reside on page1 of the SPD device.
Accessing them requires switching to page1, and adjusting the absolute
offset to be relative to the start of page1. After the access, the page
must be set back to page0. These operations are performed in several
places, so break them out into their own functions.

Also, replace a pair of default cases, which should be impossible due to
earlier checks, with __assert_unreachable().

Reviewed by: imp
MFC after: 1 week
Sponsored by: Panasas
Differential Revision: https://reviews.freebsd.org/D39842