asmc: factor out the PIO data port byte loops

[seuros]

Summary

Reuse existing pattern and evict magic numbers

No functional change: the port access sequence and the per-caller retry
and error reporting behaviour are unchanged.

[adrian]

sys/dev/asmc/asmc.c

1896–1897

I'd change these to be if (... != 0) just to be clearer.

[adrian]

all the examples in style(9) check for equality or non equality of zero. I think we should do the same here.

[seuros]

I changed the style at the end of the stack.

[adrian]

fiiiiiine