tile: fix missing unlock on error in tile_net_open()

Add the missing unlock before return from function tile_net_open()
in the error handling case.

Introduced by commit f3286a3a.
(tile: support multiple mPIPE shims in tilegx network driver)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Chris Metcalf <cmetcalf@tilera.com>
Signed-off-by: David S. Miller <davem@davemloft.net>