lan743x: remove some redundant variables and assignments

Function lan743x_phy_init assigns pointer 'netdev' but this is never read
and hence it can be removed. The return error code handling can also be
cleaned up to remove the variable 'ret'.

Function lan743x_phy_link_status_change assigns pointer 'phy' twice and
this is never read, so it also can be removed.

Finally, function lan743x_tx_napi_poll initializes pointer 'adapter'
and then re-assigns the same value into this pointer a little later on
so this second assignment is redundant and can be also removed.

Cleans up clang warnings:
drivers/net/ethernet/microchip/lan743x_main.c:951:2: warning: Value
stored to 'netdev' is never read
drivers/net/ethernet/microchip/lan743x_main.c:971:3: warning: Value
stored to 'phy' is never read
drivers/net/ethernet/microchip/lan743x_main.c:1583:26: warning: Value
stored to 'adapter' 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>