sd: Display test-perf results in MB/s, not MiB/s.

From the "SD Card Physical Layer Simplified Specification Version 3.01":
 Note that the unit of performance [MB/sec] indicates 1000x1000 [Byte/sec]
 while the unit of data size [MB] indicates 1024x1024 [Byte]. This is
 because the maximum SD Bus speed is specified by the maximum SD clock
 frequency (25 [MB/sec] = 25000000 [Byte/sec] at 50 MHz) and data size is
 based on memory boundary (power of 2).