makefs: avoid warning when creating FAT filesystem on existing file

Previously the mkfs_msdos function (from newfs_msdos) emitted warnings
in the case that an image size is specified and the target is not a
file, or no size is specified and the target is not a character device.
The latter warning (not a character device) doesn't make sense when this
code is used in makefs, regardless of whether an image size is specified
or not.

Sponsored by: The FreeBSD Foundation