ramips/rt305x: fix firmware partition size for some 4MB devices

ramips/rt305x: fix firmware partition size for some 4MB devices

allnet_all0256n-4m, tenda_w150m and unbranded_wr512-3gn-4m have
their firmware partition set to reg = <0x50000 0x3c8000>.

However, based on the 4MB flash, the size should be 0x3b0000.

After some research in the target's history, it looks like the
changed size has been a mistake when transferring device
partitions from Makefile to DTS in 770b28f146f.

This patch changes the named three devices back to 0x3b0000.

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