spi MSB first or LSB first

 LSB：least significant bit 表示二进制数据的最低位

MSB : most significant bit 表示二进制数据的最高位

spi传输数据时有两种方式：MSB first 和LSB first

例如：

ad9528 datasheet：

SPI MSB-/LSB-First Transfers
The AD9528 instruction word and payload can be MSB first or
LSB first. The default for the AD9528 is MSB first. The LSB first
mode can be set by writing a 1 to Register 0x0000, Bit 1 and Bit 6.
Immediately after the LSB first bit is set, subsequent serial control
port operations are LSB first.