spi: Remove unneded check for orig_nents

Both dma_unmap_sgtable() and sg_free_table() in spi_unmap_buf_attrs()
have checks for orig_nents against 0. No need to duplicate this.
All the same applies to other DMA mapping API calls.

Also note, there is no other user in the kernel that does this kind of
checks.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20240507201028.564630-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>