ath79: update image command for Plasma Cloud PA300

ath79: update image command for Plasma Cloud PA300

Commit 5fc28ef47959 ("ath79: Add support for Plasma Cloud PA300")
added the IMAGE/sysupgrade.bin/squashfs definition, which leaks into
other devices, resulting in sysupgrade.bin images that are actually
tarballs and do not boot when directly written to flash.

We can use the normal sysupgrade.bin command variable for this device.

Signed-off-by: Sven Wegener <sven.wegener@stealer.net>
[fix format, spelling]
Signed-off-by: David Bauer <mail@david-bauer.net>