leds: turris-omnia: use constants instead of macros for color command

Use integer constants directly when building I2C messages for LED color
change command, instead of macros. The command is simple enough to
understand what is going on even without these names.

Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>