fat: Define MAX_CLUSTSIZE using CONFIG_FS_FAT_MAX_CLUSTSIZE

Define the MAX_CLUSTSIZE to default of 65536 only if
CONFIG_FS_FAT_MAX_CLUSTSIZE is not defined.
This option has been provided to save memory in some
memory constrained cases.

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Acked-by: Michal Simek <monstr@monstr.eu>