adb tab completion: remove extraneous state

An extraneous state caused adding -s to the command line to have an
off-by-one error when trying to tab complete the device serial number.
Instead ignore that state in the state machine but still use the state
to determine when we should try to get a serial number for completion
from adb.

Bug: 22353839
Change-Id: I0c67ed6e395ea3689ba2707981c9514c6d5edabb
diff --git a/bash_completion/adb.bash b/bash_completion/adb.bash
index c1793d9..397de8e 100644
--- a/bash_completion/adb.bash
+++ b/bash_completion/adb.bash
@@ -43,9 +43,7 @@
             *)
                 if [[ $where == OPT_SERIAL ]]; then
                     where=OPT_SERIAL_ARG
-                elif [[ $where == OPT_SERIAL_ARG ]]; then
                     serial=${cur}
-                    where=OPTIONS
                 else
                     where=COMMAND
                     break