fastboot: Initialize UploadInner dsize value to 0

We must initialize UploadInner dsize value to 0, in case
stuck in ReadBuffer when receive DATA packet which dsize
is 0 or non DATA type packet. This will lead to fastboot
hang up unless USB unplugged.

Change-Id: I1e9752585c3d1013b1b1da38ead9ba4c532c2d34
Signed-off-by: qiwu chen <qiwuchen55@gmail.com>