[AVR] Fix an issue of writing 16-bit ports

For 16-bit ports, the normal devices reqiure writing high byte first
and then low byte. But the XMEGA devices require the reverse order.

Fixes https://github.com/llvm/llvm-project/issues/58395

Reviewed By: aykevl, jacquesguan

Differential Revision: https://reviews.llvm.org/D141752