zephyr: Change SPI chip select from a pointer to a struct member.

Zephyr v3.4.0 changed the SPI chip select from a pointer to a struct
member to allow using the existing SPI dt-spec macros in C++.

Signed-off-by: Maureen Helm <maureen.helm@analog.com>

ports/zephyr/machine_spi.c

@@ -102,7 +102,7 @@ mp_obj_t machine_hard_spi_make_new(const mp_obj_type_t *type, size_t n_args, siz
                 args[ARG_bits].u_int << 5 |
                 SPI_LINES_SINGLE),
         .slave = 0,
-        .cs = NULL
+        .cs = {},
     };
 
     machine_hard_spi_obj_t *self = mp_obj_malloc(machine_hard_spi_obj_t, &machine_spi_type);
@@ -157,7 +157,7 @@ static void machine_hard_spi_init(mp_obj_base_t *obj, size_t n_args, const mp_ob
         .frequency = baudrate,
         .operation = operation,
         .slave = 0,
-        .cs = NULL
+        .cs = {},
     };
 
     self->config = cfg;