fastboot: Keep borked bootloaders happy by shrinking URB len.

Some devices will fail to return the requested data if the final
URB len of the request to > 255.
So shrink the current data buffer from 128 to 127 in the acse of getting
the serial number seems to work just fine.

Change-Id: Ic24e6c86ec93f002dc721d92309b3f880f281f20