waiting for payload download on linksys_e1500_more_work

Hi @m-1-k-3,

This pull requests adds more cleanup for https://github.com/rapid7/metasploit-framework/pull/1688 and while testing I notice after asking the Linksys to download the payload, we must be sure it is downloaded before proceeding with chmod and execution. Added the code to make this wait. Please feel free to check, test, review, discuss as always :) Once you feel comfortable just merge and the original pull request will be updated, and this time yes, we'll be ready for merging :) (please test it works since I have not a true linksys device, I'm just demo'ing the testing)

thanks!

juan