This commit improves the SDHost driver for RPi3 as following:
* Unblock MMC_CMD(17). Using MMC_CMD(17) is more efficient on
* In some low probability that SEND_OP_COND might results CRC7
error. We can consider that the command runs correctly. We don't
need to retry this command so removing the code for retry.
* Using MMC_BUS_WIDTH_1 as MMC default value to improve the stability.
* Increase the clock to 50Mhz in data mode to speed up the io.
* Change the pull resistors configuration to gain more stability.
Signed-off-by: Ying-Chun Liu (PaulLiu) <email@example.com>