spl: undefined return value in spl_blk_load_image

spl_blk_load_image() should not return an uninitialized value if
blk_get_devnum_by_uclass_id() fails.

Fixes: 8ce6a2e1 ("spl: blk: Support loading images from fs")
Reported-by: Xavier Drudis Ferran <xdrudis@tinet.cat>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Xavier Drudis Ferran <xdrudis@tinet.cat>