net: amd8111e: remove redundant assignment to 'tx_index'

The variable tx_index is being initialized with a value that is never
read and re-assigned a little later, hence the initialization is redundant
and can be removed.

Cleans up clang warning:
drivers/net/ethernet/amd/amd8111e.c:652:6: warning: Value stored to
'tx_index' 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>