Replace use of _reset with _init in Xilinx platform.

[bl0x]

This looks like an oversight, when _reset was replaced with _init in FFSynchronizer.

[wanda-phi]

r+

this is an oversight, but also while we're at it, shouldn't _altera.py like... actually look at the init value?

[whitequark]

@bl0x Could you change the commit message to match our format please?

vendor._xilinx: ...

in this case.

[bl0x]

Oops, fixed.