mtd: ubi: remove redundant check on bytes_left at end of function

In function ubi_nvmem_reg_read the while-loop can only be exiting
of bytes_left is zero or an error has occurred. There is an exit
return path if an error occurs, so the bytes_left can only be
zero after that point. Hence the check for a non-zero bytes_left
at the end of the function is redundant and can be removed. Remove
the check and just return 0.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>