page-dcb: fix always true/false comparisons

Defect type: NO_EFFECT
src/connection-editor/page-dcb.c:240:unsigned_compare – This greater-than-or-equal-to-zero comparison of an unsigned value is always true. "num >= 0UL".
Defect type: NO_EFFECT
src/connection-editor/page-dcb.c:180:unsigned_compare – This greater-than-or-equal-to-zero comparison of an unsigned value is always true. "num >= 0U".
Defect type: NO_EFFECT
src/connection-editor/page-dcb.c:125:unsigned_compare – This less-than-zero comparison of an unsigned value is never true. "num < 0UL".

Found by Coverity.