FAT: buffer overflow with FAT12/16

Last commit 3831530d was intended
"explicitly specify FAT12/16 root directory parsing buffer size, instead
of relying on cluster size". Howver, the underlying function requires
the size of the buffer in blocks, not in bytes, and instead of passing
a double sector size a request for 1024 blocks is sent. This generates
a buffer overflow with overwriting of other structure (in the case seen,
USB structures were overwritten).

Signed-off-by: Stefano Babic <sbabic@denx.de>
CC: Mikhail Zolotaryov <lebon@lebon.org.ua>