tty:serial_core: Spelling mistake

fix spelling mistake "overriden" -> "overridden"

This fix resolves warning reported by checkpatch tool.

Signed-off-by: Hariprasad Kelam <hariprasad.kelam@gmail.com>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>