ramips: fix ralink_default_fw_size_16M

ramips: fix ralink_default_fw_size_16M

In ramips, there are the following predefined values for IMAGE_SIZE
ralink_default_fw_size_4M    3866624   3776k   0x3B0000
ralink_default_fw_size_8M    8060928   7872k   0x7B0000
ralink_default_fw_size_16M  16121856  15744k   0xF60000
ralink_default_fw_size_32M  33226752  32448k  0x1FB0000

Out of those, the "16M" value is obviously odd, as it provides more
room for the remaining partitions than the tree others.
Of the devices in all subtargets, there are actually > 50 that have
a firmware partition with 0xFB0000 size, while only 5 (!) have
0xF60000. From the former, many are set to
ralink_default_fw_size_16M anyway, although it is wrong at the
present point.

Consequently, it makes sense to change ralink_default_fw_size_16M
to 0xFB0000, and to update IMAGE_SIZE for the 5 devices with
0xF60000.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>