tty: vt: make rgb_from_256() slighly more comprehensible

tty: vt: make rgb_from_256() slighly more comprehensible

* make the parameter unsigned, as it is expected to be unsigned,
* make the computation easier to follow -- step-by-step, and
* don't use 85 / 2 which is only a reduced form of 255 / 6 (by a factor
  3). Unlike the former, the latter can be understood.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20240202065608.14019-2-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>