tg3: clean up redundant initialization of tnapi

tnapi is being initialized and then immediately updated and
hence the initialiation is redundant.  Clean up the warning
by moving the declaration and initialization to the inside
of the for-loop.

Cleans up clang scan-build warning:
warning: Value stored to 'tnapi' during its initialization is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>