staging: rtl8712: Remove useless variable 'fw_found'

The net device only exists when firmware is loaded successfully.
Firmware presence variable is only used through r871xu_dev_remove() and
this function already checks for net device existence.

Signed-off-by: Thomas Vegas <thomas@grouk.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>