stmmac: DMA threshold mode or SF mode can be different among multiple device instance

- In tx_hard_error_bump_tc interrupt, tc should be bumped only when current
device instance is in DMA threshold mode. Check per device xstats.threshold
other than global tc.

- Set per device xstats.threshold to SF_DMA_MODE when current device
instance is set to SF mode.

v2-changes:
- fix ident style

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: David S. Miller <davem@davemloft.net>