ARM: dts: n900: Fix offset for smc91x ethernet

Offset for smc91x must be zero otherwise smc91x linux kernel driver does not
detect smc91x ethernet hardware in qemu N900 machine.

The 0x300 offset was used to supress a warning the smsc911x
driver produces about non-standard offset as 0x300 seems to
be the EEPROM default. As only three address lines are
connected both 0 and 0x300 will work just fine with 0 being
correct. The warning about the non-standard offset can be
fixed by writing to EEPROM as that's needed in any case to
set the MAC address.

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
[tony@atomide.com: updated comments, just use 0 instead of 0x0]
Signed-off-by: Tony Lindgren <tony@atomide.com>