NET_SB1250_MAC: Update Kconfig entry

 The SB1250 network interfaces are Gigabit Ethernet ones.  Move the
Kconfig entry to the appropriate section and add some help text.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>