batman-adv: use DBG_ALL in log_level sysfs definition

Each time a new log level is added the developer must change either the DBG_ALL
enum definition and the hard coded value in the bat_sysfs.c for the log_level
attribute max value. This is extremely error prone.
With this patch the code directly uses DBG_ALL in the sysfs definition

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Sven Eckelmann <sven@narfation.org>