spi: atcspi200: Handle invalid buswidth and fix compiler warning

spi: atcspi200: Handle invalid buswidth and fix compiler warning

The kernel test robot reported a compile-time error regarding the
FIELD_PREP() value being too large for the TRANS_DUAL_QUAD field:

  error: FIELD_PREP: value too large for the field
  note: in expansion of macro 'TRANS_DUAL_QUAD'
  tc |= TRANS_DUAL_QUAD(ffs(op->data.buswidth) - 1);

This occurs because TRANS_DUAL_QUAD is defined as a 2-bit field, and
GCC's static analysis cannot deduce that `ffs(op->data.buswidth) - 1`
will strictly fall within the 0~3 range. Although the SPI framework
guarantees that `op->data.buswidth` is valid at runtime (e.g., 1, 2,
4, 8), an explicit bounds check is necessary to satisfy the compiler.

To resolve the build warning, introduce a safe fallback mechanism.
If an unexpected buswidth is encountered, the driver will trigger
a WARN_ON_ONCE to leave a trace and fall back to width_code = 0
(standard 1-bit SPI mode). This approach guarantees predictable
hardware behavior.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202602140738.P7ZozxzI-lkp@intel.com/
Suggested-by: Pei Xiao <xiaopei01@kylinos.cn>
Signed-off-by: CL Wang <cl634@andestech.com>
Link: https://patch.msgid.link/20260303024737.1791196-1-cl634@andestech.com
Signed-off-by: Mark Brown <broonie@kernel.org>