_is_mobile() returns raw strpos result, not a strict boolean

[Zodiac1978]

If match occurs at position x (int), the function returns x (and not true/false), which is a logic bug:
https://github.com/pluginkollektiv/antispam-bee/blob/master/antispam_bee.php#L2258-L2268

It works, because PHP interprets 0 as false and any int as true.