cxgb4 : Fix missing initialization of win0_lock

win0_lock was being used un-initialized, resulting in warning traces
being seen when lock debugging is enabled (and just wrong)

Fixes : fc5ab020

 ('cxgb4: Replaced the backdoor mechanism to access the HW
 memory with PCIe Window method')

Signed-off-by: Anish Bhatt <anish@chelsio.com>
Signed-off-by: Casey Leedom <leedom@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>