ide: cleanup ide_system_bus_speed()

Use the facts that:

* When ide_init() is called 'system_bus_speed' always equals zero.

* system_bus_clock() is never called before ide_init().

and:

* Move printk() from ide_system_bus_speed() to ide_init().

* Don't assign 'system_bus_speed' in ide_system_bus_speed().

* Don't call ide_system_bus_speed() in system_bus_clock().

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>