tty: hvc: remove HVC_IUCV_MAGIC

tty: hvc: remove HVC_IUCV_MAGIC

According to Greg, in the context of magic numbers as defined in
magic-number.rst, "the tty layer should not need this and I'll gladly
take patches"

This stretches that definition slightly, since it multiplexes it with
the terminal number as a constant offset, but is equivalent

Acked-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Ref: https://lore.kernel.org/linux-doc/YyMlovoskUcHLEb7@kroah.com/
Link: https://lore.kernel.org/r/8c8a2c9dfc1bfbe6ef3f3237368e483865fc1c29.1663288066.git.nabijaczleweli@nabijaczleweli.xyz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>