mmc_spi: convert timeout handling to jiffies and avoid busy waiting

SD/MMC card timeouts can be very high. So avoid busy-waiting,
using the scheduler. Calculate all timeouts in jiffies units,
because this will give us the correct sign when to involve
the scheduler.

Signed-off-by: Wolfgang Muees <wolfgang.mues@auerswald.de>
Signed-off-by: Pierre Ossman <pierre@ossman.eu>