Add KUnit tests for the __spi_map_msg() error paths. The tests verify
that a later TX or RX mapping failure clears the mapping state of
earlier transfers and leaves cur_{tx,rx}_dma_dev identifying the
current mapping device.

A zero-length transfer causes sg_alloc_table() to return -EINVAL,
providing deterministic failure injection without test hooks.
Additional cases cover successful map/unmap and a message which
requires no mapping.

Build the DMA suite as a separate translation unit, exposing the two
internal mapping helpers only for KUnit through the local internal
header. Enable SPI in the default and all-tests KUnit configurations so
the suite is exercised there.

Signed-off-by: Honghui Jiang <jiang_hh2019@163.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/20260814031419.43378-5-jiang_hh2019@163.com
Signed-off-by: Mark Brown <broonie@kernel.org>