CID1129998/1129999: Unchecked return value from library.

Check return values of various fcntl() invocations in
serialport_config(), serialport_read_nonblock() and
serialport_write_nonblock().

Also, remove some superfluous print conversion specifiers and refine
messages.

Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Acked-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>

git-svn-id: svn://flashrom.org/flashrom/trunk@1803 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1