iwlwifi: pcie: avoid dma unmap/remap in crash dump

DMA-API debug code pointed out that in this code path we
never check the return value of dma_map_page(), which could
fail.

However, we don't really even want to unmap/remap, we just
want to ensure that we can actually access the last version
of the data that the (now-dead) device may have written, so
only need to dma_sync_single_for_cpu() instead.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20210819183728.5987f35883a2.I2d9ea5ecc69a5e5947b546fb15f33363a0595651@changeid
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>