sd: do not add one sector to the disk size

This leads to random off-by-one error.
When the size of the SD is exactly 1GB, the emulation was returning a
wrong SDHC CSD descriptor.

Signed-off-by: Vincent Palatin <vpalatin@chromium.org>
Signed-off-by: Andrzej Zaborowski <andrew.zaborowski@intel.com>
