rtlbtfw(8): Fix incorrect chunk index overwrap in rtlbt_load_fwfile()
It prevented large (>256 chunks) firmwares from loading.
Fixes: 5036d9652a57 ("rtlbtfw: Firmware loader for Realtek 87XX/88XX")
Sponsored by: Future Crew, LLC
MFC after: 1 week
(cherry picked from commit a4fcbac5d891e3909474ffe7ed7064972a1a7577)