Merged in hg42/nuttx/lpc17-disable-FDR-2 (pull request #596)

disable LPC17 FDR when not used (second fix)

if a boot loader sets the fractional divider (FDR) the baud rate in
nuttx will be wrong (multiplied by this fraction).

This also has to be added to lpc17_lowputc.c

Unfortunately the constant used for shifting the bits was wrong in the header file, so it took some time to realize this...

Approved-by: Gregory Nutt <gnutt@nuttx.org>