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.
Converting phydat_t* to lsm6dsl_3d_data_t* is not correct, but it happened
to just work because the first struct member of phydat_t will be used.
Furthermore, casting &res[0] to int16_t* is wrong. Again, it just happened
to work.
