SDIO_SLAVE_FLAG_DAT2_DISABLED macro

It is required by the SD specification that all 4 data lines should be used and pulled up even in 1-bit mode or SPI mode. However, as a feature, the user can specify this flag to make use of DAT2 pin in 1-bit mode. Note that the host cannot read CCCR registers to know we don't support 4-bit mode anymore, please do this at your own risk.