IB/mthca: Use uninitialized_var() for f0

Commit 9db48926

 ("drivers/infiniband/hw/mthca/mthca_qp: kill uninit'd
var warning") added "= 0" to the declarations of f0 to shut up gcc
warnings.  However, there's no point in making the code bigger by
initializing f0 to a random value just to get rid of a warning;
setting f0 to 0 is no safer than just using uninitialized_var(), which
documents the situation better and gives smaller code too.  For example, 
on x86_64:

add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-16 (-16)
function                                     old     new   delta
mthca_tavor_post_send                       1352    1344      -8
mthca_arbel_post_send                       1489    1481      -8

Signed-off-by: Roland Dreier <rolandd@cisco.com>