sim: mask memory read/writes to access width.

Make sure that we appropriately mask the reads/writes so that they match
the RTL, otherwise this can break things like a compare of a ldr8 value
when there is garbage in the MSB's.
spi_sdcard: use a different fake CRC16 for block read.

This should really be a real CRC16, but in the mean time use a different
value - 0x55aa is a bad choice as it mirrors the bytes at the end of the
MBR.
spi_sdcard: set the idle bit for ACMD41 as used for reset polling.