intr: merge interrupt table uses of MAXCOMLEN into INTRNAME_LEN

The repeated uses of MAXCOMLEN + 1 seem a bit hazardous. If there was
a future need to change the size, the repeats will be troublesome.
Merge everything into #define INTRNAME_LEN (matches the name used by
INTRNG).

Reviewed by: markj
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D38455

(cherry picked from commit 5ad59b91535f2df176c132be10e344c4a302f619)