net: xgene: avoid bogus maybe-uninitialized warning

In some configurations, gcc cannot trace the state of variables
across a spin_unlock() barrier, leading to a warning about
correct code:

xgene_enet_main.c: In function 'xgene_enet_start_xmit':
../../../phy/mdio-xgene.h:112:14: error: 'mss_index' may be used uninitialized in this function [-Werror=maybe-uninitialized]

Here we can trivially move the assignment before that spin_unlock,
which reliably avoids the warning.

Fixes: e3978673 ("drivers: net: xgene: Fix MSS programming")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>