efi_driver: fix duplicate efiblk#0 issue

The devnum value of the blk_desc structure starts from 0,
current efi_bl_create_block_device() function creates
two "efiblk#0" devices for the cases that blk_find_max_devnum()
returns -ENODEV and blk_find_max_devnum() returns 0(one device
found in this case).

This commit uses blk_next_free_devnum() instead of blk_find_max_devnum().

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/lib/efi_driver/efi_block_device.c b/lib/efi_driver/efi_block_device.c
index add00ee..e3abd90 100644
--- a/lib/efi_driver/efi_block_device.c
+++ b/lib/efi_driver/efi_block_device.c
@@ -124,10 +124,8 @@
 	struct efi_block_io *io = interface;
 	struct efi_blk_plat *plat;
 
-	devnum = blk_find_max_devnum(UCLASS_EFI_LOADER);
-	if (devnum == -ENODEV)
-		devnum = 0;
-	else if (devnum < 0)
+	devnum = blk_next_free_devnum(UCLASS_EFI_LOADER);
+	if (devnum < 0)
 		return EFI_OUT_OF_RESOURCES;
 
 	name = calloc(1, 18); /* strlen("efiblk#2147483648") + 1 */