ANDROID: usb: f_accessory: Remove useless assignment

acc_alloc_inst() assigns to a local 'dev' variable, but then never uses
it. Remove the redundant assignment, and the local variable along with
it.

Bug: 173789633
Signed-off-by: Will Deacon <willdeacon@google.com>
Change-Id: Ide9c2e89fb12b846eb8739b302d1b742fc7eb6b5
Signed-off-by: Giuliano Procida <gprocida@google.com>