staging: fbtft: fbtft-core: Fix last line displayed on fbcon

staging: fbtft: fbtft-core: Fix last line displayed on fbcon

For the special case when fbtft_mkdirty() is called with with -1 for the y
coordinate, the height is truncated by 1.

This isn't required, and causes the last line to not update.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20191011112441.31003-1-alexandru.ardelean@analog.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>