Staging: batman-adv: Dont deactivate aggregation on wrong input

A non-integer changes the aggregation mode. Therefore this patch changes
the behaviour to explicitly check strict_strtoul()'s return code.

Signed-off-by: Linus Lüssing <linus.luessing@web.de>
Acked-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>