drm/panel: s6e63m0: Simplify SPI writing

This writing code is equivalent to the spi_write()
helper in the SPI framework. Insert a comment
that this will always work fine since SPI buffers
are in native endianness.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Stephan Gerhold <stephan@gerhold.net>
Cc: Paweł Chmiel <pawel.mikolaj.chmiel@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201110234653.2248594-2-linus.walleij@linaro.org