Fix stop condition when receiving 1 byte with USCI

According to MSP430x2xxx family guide, page 473:

If a master wants to receive a single byte only, the UCTXSTP bit must be
set while the byte is being received. For this case, the UCTXSTT may be
polled to determine when it is cleared:

BIS.B #UCTXSTT,&UCBOCTL1 ;Transmit START cond.
POLL_STT BIT.B #UCTXSTT,&UCBOCTL1 ;Poll UCTXSTT bit
JC POLL_STT              ;When cleared,
BIS.B #UCTXSTP,&UCB0CTL1 ;transmit STOP cond.

Thread on 43oh: http://forum.43oh.com/topic/3154-i2c-issues/
Merge pull request #226 from astuder/master

Fix I2C stop condition when receiving 1 byte with USCI