core: update usbi_dbg to take the context as an argument

core: update usbi_dbg to take the context as an argument

This commit fixes a performance issue caused by the disconnection of the first context
allocated from the default context. usbi_dbg now takes the explicit context instead of
relying on the default context (which may not exist) in most cases. All call sites have
been updated to pass the context or explicitly pass NULL if the context is not
available. We should actively discourage using NULL as the context in the future and
patch all call sites to always pass the context.

Fixes #951

Signed-off-by: Nathan Hjelm <hjelmn@google.com>