ethernet: ionic: Fix DMA mapping tests

ethernet: ionic: Fix DMA mapping tests

Change error values of `ionic_tx_map_single()` and `ionic_tx_map_frag()`
from 0 to `DMA_MAPPING_ERROR` to prevent collision with 0 as a valid
address.

This also fixes the use of `dma_mapping_error()` to test against 0 in
`ionic_xdp_post_frame()`

Fixes: 0f3154e6bcb3 ("ionic: Add Tx and Rx handling")
Fixes: 56e41ee12d2d ("ionic: better dma-map error handling")
Fixes: ac8813c0ab7d ("ionic: convert Rx queue buffers to use page_pool")
Signed-off-by: Thomas Fourier <fourier.thomas@gmail.com>
Reviewed-by: Brett Creeley <brett.creeley@amd.com>
Link: https://patch.msgid.link/20250619094538.283723-2-fourier.thomas@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>