pytests: fix typo in removable_storage test

The variable should be _bft_fixture instead of bft_fixture

BUG=none
TEST=make sure USB tests works in factory test UI

Change-Id: Ie891bf845c60e53d087bddaeb47f3a1c7d1cc13c
Reviewed-on: https://chromium-review.googlesource.com/225543
Reviewed-by: Ricky Liang <jcliang@chromium.org>
Commit-Queue: Wei-Ning Huang <wnhuang@chromium.org>
Tested-by: Wei-Ning Huang <wnhuang@chromium.org>
1 file changed