boot: set correct block device name in set_efi_bootdev()

For SATA devices the class name is 'ahci' but the block device name is
'sata'.

Use function blk_get_uclass_name() to retrieve the correct string.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>