realtek: 6.6: harden fw_init_cmdline()

realtek: 6.6: harden fw_init_cmdline()

Some devices (e.g. HP JG924A) hand over other than expected kernel boot
arguments. Looking at these one can see:

fw_init_cmdline: fw_arg0=00020000
fw_init_cmdline: fw_arg1=00060000
fw_init_cmdline: fw_arg2=fffdffff
fw_init_cmdline: fw_arg3=0000416c

Especially fw_arg2 should be the pointer to the environment and it looks
very suspicous. It is not aligned and the address is outside KSEG0 and
KSEG1. Booting the device will result in a hang. Do better at verifying
the address.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Suggested-by: Bjørn Mork <bjorn@mork.no>