block: mtip32xx: Fix usage of dma_map_sg()

The dma_map_sg() can fail and, in case of failure, returns 0.  If it
fails, mtip_hw_submit_io() returns an error.

The dma_unmap_sg() requires the nents parameter to be the same as the
one passed to dma_map_sg(). This patch saves the nents in
command->scatter_ents.

Fixes: 88523a61 ("block: Add driver for Micron RealSSD pcie flash cards")
Signed-off-by: Thomas Fourier <fourier.thomas@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20250627121123.203731-2-fourier.thomas@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>