cuda: fix len calculation in cuda_adb_req

When advancing buffer by 1, len should be shorthened by appropriate
amount, instead of setting it to -1.

Found with cppcheck.

Signed-off-by: Amadeusz Sławiński <amade@asmblr.net>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>