sdcard: Compute CRC7 for all SPI commands.

Some SD cards (e.g. SDXC) have CRC checking permanently enabled in SPI
mode. Sending commands with CRC=0x00 causes them to return ILLEGAL_COMMAND,
making initialisation fail. Fix this by computing a valid CRC7 for every
command.

Signed-off-by: Kwabena W. Agyeman <kwagyeman@live.com>