lib/ts_bm: add helper to reduce indentation and improve readability

The flow-control of `bm_find` is very deeply nested with a conditional
comparing a ternary expression against the pattern inside a for-loop
inside a while-loop inside a for-loop.

Move the inner for-loop into a helper function to reduce the amount of
indentation and make the code easier to read.

Fix indentation and trailing white-space in preceding debug logging
statement.

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Florian Westphal <fw@strlen.de>