scripts: compliance: checkpatch: Fix regex

The checkpatch regex matched too greedily, so that descriptions with ":"
would match too much, example:

-:20: ERROR:SPACING: spaces required around that '=' (ctx:VxW)

Would match "SPACING: spaces required around that '=' (ctx" instead of
just "SPACING".

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>