Fix: Static payload size bug prev introduced

Introduced a bug by changing payload_size value to a static value of 32
and forgot to revert it. The commit was:

https://github.com/TMRh20/RF24/commit/aac1650646b2c440c8822deb2f99514f4e2e4ddd#diff-6dbdcee25d60de0476cb76000ce10fd3

Discovered due to #12 - Issue submitted by cbolanos79