serial: fix up uart_poll_in docstring

The uart_poll_in() Doxygen comment has a typo ("if empty" should be "is
empty"). Fix it up and make it more clear.

Change-Id: I222051dedc6036e70ce94e8046084d763628ff13
Signed-off-by: Marti Bolivar <marti.f.bolivar@gmail.com>