Add attribute to show the current clock source for the serial
driver and remove old and annoying debug output.

Note, this only currently shows the current source with a
"* " prefix to indicate that it is the current source. Future
code will list all the clock sources, with the non-selected
one with "  " prefix.
Signed-off-by: Ben Dooks <ben-linux@fluff.org>
PATCH FOLLOWS
KernelVersion: 2.6.26-rc3