access: jack: simplify previous power of 2 calculation
Note that there is a slight change in behaviour here, before if i_read was 1 greater than a power of 2 (e.g., 257), it would evaluate to the previous previous power of 2 (e.g., 128). Now in those cases it will really be the previous power of 2 (e.g., 256).
Edited by Tristan Matthews