drivers: spi: fix CS documentation

`spi_cs_control` `cs` field is no longer a pointer, but a struct member.
Clarify it must be zero-initialized if not used. Remove confusing
comment about the need to use `device_is_ready()` on `cs` field,
`spi_is_ready_dt()` is the only thing users need to do.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>