adb: Fix compiler warning

system/core/adb/adb.c: In function 'connect_device':
system/core/adb/adb.c:1001: warning: comparison between signed and unsigned integer expressions

Change-Id: I206f85395e1d7ad8d6ef130a26c95dcf0f498696
Signed-off-by: Scott Anderson <saa@android.com>