fastboot: windows: fix Read function

Read function exits before reading all requested bytes. Fix that by
looping until the len requested is completed.

This will fix the issue execting get_staged command for a staged image
bigger than 1MB

Test: fastboot get_staged file.txt
Change-Id: Ic70ab48f3a8c8d78c225db638892501d4dc20b13
Signed-off-by: Fernando Lugo <flugo@google.com>