loader/libofw: Fix disk size truncation

At present OF_ioctl first multiplies, then casts to 64-bit, meaning at
the asm level it truncates the result to 32-bit, then zero-extends it to
64-bit to return. Cast n to 64-bit before multiplying, so that the
correct result is returned.

(cherry picked from commit cd6e526e268e4fdf1c9a65b9d792e67343f52307)