serial: samsung: improve code clarity by defining a variable

The of_node is derived from pdev for every usage, define a
device_node variable instead.

Signed-off-by: Naveen Krishna Chatradhi <ch.naveen@samsung.com>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>