serial: samsung: Fix build error

Fixes the following build error due to a typo introduced
by commit e4ac92df ("serial: samsung: Neaten dbg uses"):
drivers/tty/serial/samsung.c:69:26: error: ‘buf’ undeclared (first use in this function)

Reported-by: Arun Kumar K <arun.kk@samsung.com>
Signed-off-by: Sachin Kamat <sachin.kamat@samsung.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Christian Engelmayer <cengelma@gmx.at>
Acked-by: Joe Perches <joe@perches.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>