fs: fat: fix file_fat_detectfs()

Up to now file_fat_detectfs() did not detect some interface types like
EFI, HOST, VIRTIO.

Avoid duplicate code by calling blk_get_if_type_name().

The interface type now will be shown in lower case to match all other use
cases.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>