libbladeRF: Decreased verbosity of library outputs

In the future, we'll need to standardize the situations in which we use
the log_* functions.  For the time being, it is preferred to err on the
side of less verbosity.

Here's a preliminary list of intended usages:

log_error():
    Unexpected and severe failures.

log_warning():
    Failures and situations that may yield unexpected behavior (e.g.,
    truncation)

log_info(), log_debug(), log_verbose():
    Information about what's going on under the hood. log_info() is the
    least verbose, log_verbose is the most.