lan743x: remove redundant initialization of variable current_head_index

The variable current_head_index is being initialized with a value that
is never read and it is being updated later with a new value.  Replace
the initialization of -1 with the latter assignment.

Addresses-Coverity: ("Unused value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>