vfat: Fix mkcksum argument sizes

In case a function argument is known/fixed size array in C, the argument is
still decoyed as pointer instead ( T f(U n[k]) ~= T fn(U *n) ) and therefore
calling sizeof on the function argument will result in the size of the pointer,
not the size of the array.

The VFAT code contains such a bug, this patch fixes it.

Reported-by: Aaron Williams <Aaron.Williams@cavium.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Tom Rini <tom.rini@gmail.com>
Cc: Aaron Williams <Aaron.Williams@cavium.com>
Tested-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>