subsys/mgmt/hawkbit: optimize log message

A lot of the log messages are using highest level, optimize
them to recycle as much messages as possible.

When building the sample with `CONFIG_HAWKBIT_LOG_LEVEL_DBG`
on `frdm_k64f`, the FLASH size is reduced from 150860 B to
150048 B, saving ~800 bytes.

Signed-off-by: Yong Cong Sin <[email protected]>

Author: ycsin