media: dvb_frontend: don't play tricks with underflow values

fepriv->auto_sub_step is unsigned. Setting it to -1 is just a
trick to avoid calling continue, as reported by Coverity.

It relies to have this code just afterwards:

	if (!ready) fepriv->auto_sub_step++;

Simplify the code by simply setting it to zero and use
continue to return to the while loop.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>