efi_loader: fix EFI_FILE_PROTOCOL.GetInfo()

We check the existence of files with fs_exist(). This function calls
fs_close(). If we do not set the active block device again fs_opendir()
fails and we do not set the flag EFI_FILE_DIRECTORY. Due to this error the
`cd` command in the EFI shell fails.

So let's add the missing set_blk_dev(fh) call.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>