sdhci-bcm2708: raise DMA sync timeout

Commit d64b84ca by accident reduced the maximum overall DMA sync
timeout. The maximum overall timeout was reduced from 100ms to 30ms,
which isn't enough for many cards. Increase it to 150ms, just to be
extra safe. According to commit 872a8ff7 in the MMC subsystem, some
cards require crazy long timeouts (3s), but as we're busy-waiting,
and shouldn't delay for such a long time, let's hope 150ms will be
enough for most cards.