PIXMA: Decode SCAN button interrupt on MG3200 series
The MG3200 series reports the SCAN button in buf[4] of the interrupt packet instead of buf[0], so button-controlled scan never started. Decode it and set PIXMA_EV_BUTTON1.
The MG3200 series reports the SCAN button in buf[4] of the interrupt packet instead of buf[0], so button-controlled scan never started. Decode it and set PIXMA_EV_BUTTON1.