Improve interrupt handling for USCI.

Because some interrupts share an interrupt vector, we use the relevant interrupt
enable bit to decide if any user is interested in the interrupt.  If not, that
interrupt is skipped so that another interrupt in the if/then/elseif chain can
be taken.  We also defensively try to prevent I2C interrupts from being a
problem if not used, although no I2C code using these interrupts has yet been
written.