fat: fix FAT sector offsets overflow on large FAT partitions

This patch fixes several issues where sector offsets can overflow due
to being limited to 16-bits. The cases where an overflow can happen
when accessing large FAT32 partitions are:

 - length of FAT in sectors
 - start sector of root directory
 - the sector of the first cluster

These issues were observed when reading files from a 64GB FAT32
filesystem.

Signed-off-by: Aaron Williams <aaron.williams@caviumnetworks.com>
Tested-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Anatolij Gustschin <agust@denx.de>