The URXE and UTXE module enables for SPI mode on the MSP430x337
are combined into one single module enable USPIE on the MSP430x337A.
Another important difference is that in the `337A the TXEPT bit
in the UTCTL register should not be reset by user software. It
is recommended to use a bis.b instruction instead of using a mov.b
instruction to program the required selection bits in the UTCTL
register to avoid unintentional resetting of the TXEPT bit.
This incompatibility is due to bug fixes done in the `A` version.