package/rpi-wifi-firmware: bump version to 883b726

Changelog:
  - Update CYW43455 bluetooth firmware,
    Second Spectra fix for CYW43455 (CVE-2020-10370)

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>