xec: spi: Remove `.cs = NULL` in spi_config initializer

Following #56576, the `cs` field in `struct spi_config` is of type
`struct spi_cs_control` instead of a pointer to the same type.
`spi_xec_qmspi_ldma.c:qmspi_xec_init` tries assigning `NULL` to the
`.cs` field through a designated initializer, which causes a compilation
error.

This PR simply removes the `.cs = NULL` line. The designated initializer
will automatically zeroize the underlying GPIO pin info, which should
have the same effect that setting the pointer to NULL did previously.

Signed-off-by: Tristan Honscheid <honscheid@google.com>