staging: iio: tsl2583: cleaned up logging

There are several places in the code where the function name is
hardcoded in the log message. Use the __func__ constant string to build
the log message. This also clarifies some of the error messages to match
the code and ensures that the correct priority is used since the message
is already being changed.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>