thunderbolt: Use min() for the DMA path credit cap

tb_dma_reserve_credits() caps the request against what the adapter has
left by decrementing one credit at a time. The other arm of the same
if() already caps with min(port->total_credits, credits); use min()
here too.

No functional change: the object code is unchanged.

Assisted-by: Claude:claude-opus-5
Signed-off-by: Fan Ye <fy15309206903@gmail.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>