drivers: spi: spi_pico_pio: Prevent dma_config check if DMA is not enabled

At this moment, DMA for SPI 3-wire half-duplex operation is not supported
by the pio driver. A check was implemented there to prevent user from
enabling DMA, but wasn't bypassed when CONFIG_SPI_RPI_PICO_PIO_DMA is
not enabled at all, under which the `dma_config` structure isn't defined
at all.

Fixed #94897.

Signed-off-by: Terry Geng <terry@terriex.com>