drivers: net: xgene: Simplify xgene_enet_setup_mss() to kill warning

With gcc-4.1.2 and -Os:

    drivers/net/ethernet/apm/xgene/xgene_enet_main.c: In function ‘xgene_enet_start_xmit’:
    drivers/net/ethernet/apm/xgene/xgene_enet_main.c:297: warning: ‘mss_index’ may be used uninitialized in this function

Using a separate variable to track success may confuse the compiler.
Preinitialize mss_index with -EBUSY and check for negative error values
instead to kill the warning.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: David S. Miller <davem@davemloft.net>