`transceivers` should check `flat_mem`

[Aaron-Hartwig]

SFF-8636 has a bit called flat_mem to indicate whether or not a module supports paging.

Right now when we receive a message from the host we blindly write the page select register to whatever the host is asking for (for SFF-8636, this is nearly always page 0 anyway). However, we've observed with at least one module that writing to page select when paging may not be supported results in a NACK. We should learn if a module has a flat memory map or not and use that to decide if we will write to page select on every operation or not.