net: tg3: avoid uninitialized variable warning

The tg3_set_eeprom() function correctly initializes the 'start' variable,
but gcc generates a false warning:

drivers/net/ethernet/broadcom/tg3.c: In function 'tg3_set_eeprom':
drivers/net/ethernet/broadcom/tg3.c:12057:4: warning: 'start' may be used uninitialized in this function [-Wmaybe-uninitialized]

I have not come up with a way to restructure the code in a way that
avoids the warning without making it less readable, so this adds an
initialization for the declaration to shut up that warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>