V4L/DVB (8252): buf-dma-sg.c: avoid clearing memory twice

1) Remove a useless initialisation of 'i'

2) Avoid clearing the memory allocated twice (once in 'kcalloc', once
   in 'sg_init_table')

3) Remove a test that can never trigger.  The function returns NULL in
   such a case, so we know that at this point 'pages[0]' != NULL

Signed-off-by: Christophe Jaillet <jaillet.christophe@wanadoo.fr>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>