[media] dvb: earth-pt1: decrease the too large DMA buffer size

Current default value of "nr_tables" option corresponds to the DMA
buffer of about 10 to 48 seconds long, which is obviously too much.

Signed-off-by: Akihiro Tsukada <tskd2@yahoo.co.jp>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>