serial: sc16is7xx: improve comments about variants

Replace 740/750/760 with generic terms like 74x/75x/76x to account for
variants like 741, 752 and 762.

Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Reviewed-by: Lech Perczak <lech.perczak@camlingroup.com>
Link: https://lore.kernel.org/r/20230905151300.15365-1-hugo@hugovil.com


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