drivers: spi: silabs: Add missing iodev_submit implementation

Series 2 EUSART and SiWx91x GSPI drivers were missing the
`iodev_submit` member from their API structs, leading to
null pointer dereference if CONFIG_SPI_RTIO was used.

Signed-off-by: Aksel Skauge Mellbye <aksel.mellbye@silabs.com>