p54usb: remove some dead code

Since "p54: prevent upload of wrong firmwares" we no longer allow
outdated LM86 firmwares to be uploaded on ISL3887 (LM87) devices.
Therefore we can purge this buggy legacy code altogether.

Signed-off-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>