staging: fbtft: fix voltage settings for C-Berry28

This patch fixes some internal voltage settings for the C-Berry28 display.
The original example source files for the C-Berry28 as provided by
its vendor admatec contained six issues where a command parameter's value
didn't match its corresponding comment.
I've informed admatec about these discrepancies on 2015-08-25. In the
meantime, I've assumed the comments to be more reliable than the code,
and thus have used these values to write the initial C-Berry28 driver.
After having received a reply from admatec on 2015-10-16 that the issues
have been fixed in their example code, it has turned out that for the
voltages VCOM and AVDD, the code was indeed correct, but the comments
were wrong. This patch is meant to fix these two pending mistakes.

Signed-off-by: Dennis Menschel <menschel-d@posteo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>