bpf: fix BTN_STYLUS for the XP Pen ACK05 remote

Usage_Dig_BarrelSwitch was applied in the UsagePage_Button which incorrectly mapped to BTN_TOOL_PENCIL

Closes #60 (closed)

Fixes: 4df79b56 ("bpf: add a v6.11+ compatible BPF fixup for the XPPen ACK05 remote")

cc @dataCobra, @bentiss

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Fixing this in stable/ though technically that's a bit of a no-no. For something that's a clear bug that seems acceptable? :)