scanimage_pieusb: fix to link with backend
pieusb: cmdDoInquiry -> pieusb_cmd_inquiry
pieusb: cmdGetShadingParameters -> pieusb_cmd_get_shading_parms
pieusb: commandScannerRepeat -> pieusb_command
Use SCSI_COMMAND_LEN everywhere

Use it to loop over scsi cmd bytes and check result after every write.
pieusb_usb: improve status checks on command/data i/o
pieusb: clean up call to sanei_usb_find_devices()
pieusb_usb: improve error reporting after timeout

actually detect SCSI_STATUS_TIMEOUT
properly catch and report failed states
pieusb: set USB timeout to 10 sec