drivers: spi: xmc4xxx: Fix comp with interrupt enabled and dma disabled

When dma is enabled, spi_xmc4xxx_transcieve_dma() needs to disable the
isr used by spi_xmc4xxx_transceive(). Renable the isr at the
end of spi_xmc4xxx_transceive_dma() instead of in spi_xmc4xxx_transceive().
Doing this in latter function will fail compilation when interrupt
support is enabled but dma is disabled.

This is regression from 8494b641
but was only caught when xmc47_relax_kit was added which tested this
scenario in tests/drivers/spi/spi_loopback.

Fixes #57494

Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>