Noticed by Marcel Cotta <mc123@mail.ru>.

ide_init() always initializes system_bus_speed variable
so system_bus_clock() should never call ide_system_bus_speed()
and no driver is calling ide_system_bus_speed() directly.

Bug was that if no IDE kernel parameter was given during boot
system_bus_speed will be zeroed in init_ide_data().

This patch should fix the problem
(as a bonus -> no need to zero these variables they are static).