This makes it easier to use common error reporting such as `strerror()`
or `tiny_strerror()` to give more insight on why something failed.
The custom error codes via `enum` have been updated to be synonymous
with the `errno` codes for backward compatibility.
In addition, `pcf857x_init()` has been updated to no longer or
together the return code, but rather abort on the first fail transaction
and return the error code as is. Otherwise (when both fail due to
different error codes) the returned error code may be garbage.
This allows including the header without using the module. Obviously,
calls to the functions provided by the header won't like without using
the module. But including the header can still be useful for e.g.:
if (IS_USED(MODULE_PCF857x)) {
/* make use of the module */
}
In the above example all calls to pcf857x functions would be optimized
out when the module is not used, full compile checks happen in either
case.
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.
