serial: sc16is7xx: refactor FIFO access functions to increase commonality

Simplify FIFO access functions by avoiding to declare
a struct sc16is7xx_port *s variable within each function.

This is mainly done to have more commonality between the max310x and
sc16is7xx drivers.

Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Link: https://lore.kernel.org/r/20231221231823.2327894-15-hugo@hugovil.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>