logging: fix timestamp func overwrite on log2

When LOG2 is enabled, timestamp func which was just set
according to various conditions, is overwritten using sysclock.
Since sysclock is very fast, it will cause uint32_t to wrap very
fast and cause the time to start over from 0 often.

This commit combines the two statements doing the same thing,
to one statement.

Signed-off-by: Emil Lindqvist <emil@lindq.gr>