msp430 adc12: turn off warning about bitfield code gen

adc12, usart, and usci bitfield code effected.  Just turn the
warnings off.   Bitfields work fine.  Been using the bitfield
code generation in 4.6.3 for some time now.  Used in active
code for ADC12, the USART in the 1611 code, and the USCI code
for the 2618 code.