usb: gadget: adb: allow freezing in adb_read

wait_event_interruptible in adb_read might return -ERESTARTSYS if
userspace is frozen during adb_read or another signal is delivered
to adb.  If so, don't set dev->error to avoid resetting the adb
connection.

Change-Id: I5a7baa013a9a3a3b5305de7e6a0d18546a560018
Signed-off-by: Colin Cross <ccross@android.com>