#97 Initial MMC idle polling too short

[Harald Kipp]

When plugging in a new card the initialization sometimes fails with specific cards (e.g. Transcend 1G SD).

It turns out that cards may need more than half second to respond to SEND_OP_CODE after initial reset. In Nut/OS 4.8 and earlier the driver gives up after about 0.4s.

Furthermore, if the card has been initialized, it typically responds on the first trial, so there is no need to poll MMC_MAX_INIT_POLLS/4 without delay. Using a sleep for 10ms after each failed poll should provide a good compromise between lowest polling rate and fastest detection. Some cards respond in less than 0.2s.

The default value of MMC_MAX_INIT_POLLS may be reduced from 512 to 256.