mmc: atmel_sdhci: not on capabilities to set gck rate

mmc: atmel_sdhci: not on capabilities to set gck rate

The capabilities have default values which doesn't reflect the reality
when it concerns the base clock and the mul value. Use a fixe rate
for the gck. 240 MHz is an arbitrary choice, it is a multiple of the
maximum SD clock frequency handle by the controller and it allows to
get a 400 kHz clock for the card initialisation.

Signed-off-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Signed-off-by: Wenyou Yang <wenyou.yang@microchip.com>