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>