This makes life easier when calling e.g. `saul_reg_write()` with data
stored in flash.
As now the signatures for reading and writing differ (in that `const`
qualifier only), `saul_notsup()` is split into `saul_write_notsup()`
and `saul_read_notsup()`. However, one is implemented as a symbol alias
of the other, so that ROM consumption remains unchanged.
All DS18 functions have a dev argument. All except the init function use
it as an IN parameter, so we can prototype it as const ds18_t*.
As a consequence we can drop the cast in read_temperature() in ds18_saul.c
which was the primary trigger for the changes.
The commit also follows the preferred convention that "params" is a field
in the device struct. Only the init function needs to write it.
