fix bad return value in dhcp_do_request_*

1. bad return status make it return uninitialized prefixLength with a
magic
number in android_net_utils_runDhcpCommon()
2. it will make DhcpInfoInternal.makeLinkAddress() to throw
IllegalArgumentException when try to connect a Wifi AP
3. the exception will make system reboot, of cource it random.

Change-Id: Iabc0cd5b69c1316f4eeae0aa1b8a91444dca4562
Signed-off-by: Michael Wu <michaelstay@gmail.com>