Bluetooth: ipsp: use sys_log

Various of the printk messages in this sample app don't have
newlines. Let's fix the output by moving to SYS_LOG_xxx() macros
instead, which don't need them anyway.  This also makes it easier to
tell errors apart from other messages, etc.

Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>