urtps: fix UART speed setting to work in Linux

The previous code used the numeric baudrate as the speed argument
for cfsetispeed() and cfsetospeed(). This works in OSX and NuttX,
but is not portable.

This fix introduces a new method to convert baudrate to speed using
constants such as B115200.

Signed-off-by: Gabriel Moreno <gabrielm@cs.cmu.edu>