media: aspeed: Correct values for detected timing

Correct timing's fp/sync/bp value based on the information below.
It should be noticed that the calculation formula should be changed
per sync polarity.

The sequence of signal: sync - backporch - video data - frontporch

The following registers start counting from sync's rising edge:
1. VR090: frame edge's left and right
2. VR094: frame edge's top and bottom
3. VR09C: counting from sync's rising edge to falling edge

[Vertical timing]
            +--+     +-------------------+     +--+
            |  |     |     v i d e o     |     |  |
         +--+  +-----+                   +-----+  +---+
       vsync+--+
   frame_top+--------+
frame_bottom+----------------------------+

                  +-------------------+
                  |     v i d e o     |
      +--+  +-----+                   +-----+  +---+
         |  |                               |  |
         +--+                               +--+
       vsync+-------------------------------+
   frame_top+-----+
frame_bottom+-------------------------+

[Horizontal timing]
            +--+     +-------------------+     +--+
            |  |     |     v i d e o     |     |  |
         +--+  +-----+                   +-----+  +---+
       hsync+--+
  frame_left+--------+
 frame_right+----------------------------+

                  +-------------------+
                  |     v i d e o     |
      +--+  +-----+                   +-----+  +---+
         |  |                               |  |
         +--+                               +--+
       hsync+-------------------------------+
  frame_left+-----+
 frame_right+-------------------------+

Ex. 1920x1200@60 whose vsync polarity is negative
  VR098: c4d3efff, VR09C: 04cc001f
  v-total = 0x4D3 (VR098[27:16]) = 1235
  v-sync  = 0x4CC (VR09C[27:16]) = 1228

[hverkuil: drop unused variable mds]

Signed-off-by: Jammy Huang <jammy_huang@aspeedtech.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>