efi_loader: Exit in efi_set_bootdev() upon invalid "desc"

When trying to load an image from a non-existent USB key, U-Boot v2017.11
crashes on my x86 platform:

=> load usb 0:1 03000000 abc
General Protection
EIP: 0010:[<7b59030d>] EFLAGS: 00010286
Original EIP :[<fff4330d>]
...

This used to work in v2017.09. Testing has shown, that this bug was
introduced with patch 95c5553e [efi_loader: refactor boot device and
loaded_image handling].

This patch now checks if a valid "desc" is returned from blk_get_dev()
and only continues when "desc" is available. Resulting in this cmd
output (again):

=> load usb 0:1 03000000 abc
** Bad device usb 0 **

Signed-off-by: Stefan Roese <sr@denx.de>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>