serial: omap-serial: fix signess error

"ret" is unsigned, so check for (ret < 0) made no sense.
Made it signed.

Signed-off-by: Vasiliy Kulikov <segooon@gmail.com>
Acked-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>