loader: always set media size from partition.
The disk access is validated by using partition table definitions, therefore
we have no need for if statements, just set the disk size.
Of course the partition table itself may be incorrect/inconsistent, but if
so, we are in trouble anyhow.
Testing done: this patch has been on loader branch since march 15 2017, it has been in every image I have built since. No related issues reported. Also this one + next major biosdisk patch (due to dependency tree) have specifically been tested in some “weird” behaving systems and finally, I did commit it in FreeBSD Current https://svnweb.freebsd.org/base?view=revision&revision=340215