Clean this up a bit. Use register names that match the documentation, and

Clean this up a bit.  Use register names that match the documentation, and
fix some register number.  Drop reading the Interrupt Pending register
in the interrupt handler as this was actually reading the Interrupt Enable
register and the bits that were checked were always true.  Fix bogus
code in sfuartcnputc() and simplyfy sfuartcngetc().  Throw in some
style fixes for good measure.