Make the iSerialNumber string read robust to slow or failing devicesHEADmaster

libusb_get_string_descriptor_ascii() uses a fixed 1 s timeout for the
control transfer. The Apple T1 (iBridge1,1) in recovery mode answers the
iSerialNumber read only intermittently and often later than that, so the
read returns LIBUSB_ERROR_TIMEOUT. Losing it costs the device identity and
the AP and SEP nonces, and in the hotplug add handler it meant giving up on
the device: IRECV_DEVICE_ADD was never emitted and callers concluded that
nothing was attached.

- When the libusb helper fails, retry the descriptor read a few times with
  libirecovery's own USB_TIMEOUT.
- On Linux, when the read still yields no iBoot string, fall back to the
  serial string the kernel already cached for the same bus and device
  number under /sys/bus/usb/devices. Used both when opening a device and in
  the hotplug add handler; never fatal, and a no-op on other platforms.
- Only assume the legacy CPID 0x8900 when the iBoot string is non-empty but
  carries no CPID field. An empty string means the read failed, and
  claiming 0x8900 there mis-identifies every modern part.

Behaviour for devices that answer the first read promptly is unchanged.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

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