V4L/DVB (12206): get_dvb_firmware: Correct errors in MPC718 firmware extraction logic

The extraction routine for the MPC718 "firmware" had 2 bugs in it, where one
bug masked the effect of the other.  The loop iteration should have set
$prevlen = $currlen at the end of the loop, and the if() check should have used
&& instead of || for deciding if the firmware length is reasonable.

Signed-off-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>