via-cuda: Remove redundant temporary variable

There is no possibility that current_req can change during execution of
cuda_start(). This can be confirmed by inspection: cuda_lock is always
held whenever cuda_start() is called or current_req is modified.

Tested-by: Stan Johnson <userm57@yahoo.com>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>