video/smscufx: fix line counting in fb_write

Line 0 and 1 were both written to line 0 (on the display) and all subsequent
lines had an offset of -1. The result was that the last line on the display
was never overwritten by writes to /dev/fbN.

The origin of this bug seems to have been udlfb.

Signed-off-by: Alexander Holler <holler@ahsoftware.de>
Cc: stable@vger.kernel.org
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>