accel/amdxdna: Fix iommu_map_sgtable() return value handling

accel/amdxdna: Fix iommu_map_sgtable() return value handling

iommu_map_sgtable() returns negative error codes on failure, but the
result is stored in an unsigned variable. This prevents proper error
detection.

Change the variable type to ssize_t so negative error values can be
handled correctly.

Fixes: ece3e8980907 ("accel/amdxdna: Allow forcing IOVA-based DMA via module parameter")
Reported-by: Dan Carpenter <error27@gmail.com>
Closes: https://lore.kernel.org/all/adk7kOUBwIyYnX1M@stanley.mountain/
Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org>
Signed-off-by: Wendy Liang <wendy.liang@amd.com>
Signed-off-by: Lizhi Hou <lizhi.hou@amd.com>
Link: https://patch.msgid.link/20260413180238.668441-1-lizhi.hou@amd.com