cmd_ubi: use int64_t volume size for 'ubi create'

int64_t matches the bytes field in struct ubi_mkvol_req to which the
size is assigned. With the prior signed 32 bit integer, volumes were
restricted to being less than 2GiB in size.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Acked-by: Stefan Roese <sr@denx.de>