usb: mass_storage: rework and cleanup logging

This patch reworked logging so that the logging levels are more
finely graded and the locations with non-critical error handling
(below usb_ep_set_stall's) are logged with SYS_LOG_WRN.
Also cleanup, add log domain and fix indents.

Signed-off-by: Johann Fischer <j.fischer@phytec.de>