serial: sh-sci: Improve type-safety calling sci_receive_chars()

While ptr and port both point to the uart_port structure, the former is
the untyped pointer cookie passed to interrupt handlers.
Use the correctly typed port variable instead, to improve type-safety.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Ulrich Hecht <uli+renesas@fpond.eu>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>